TB Disasters -- Slipped Camshaft Sprocket

Franko6 · Jul 10, 2008

Yesterday I pulled a marathon for a poor guy that followed all the rules for an ALH, did a decent Timing Belt changout using one of the popular vendors timing belt kits. He followed all the instructions on the Timing Belt Replacement instructions in the TDIClub "Articles" section. The Bentley Torque specifications were followed and the work done accurately.

The car went 15 miles and the timing belt circuit failed, breaking two lifters and bending multiple valves. Cause of the failure: the camshaft sprocket came loose from the camshaft.

The 'correct' torque setting of 45Nm or 33 ft lbs was applied. That torque specification is the point of failure for the engine I worked on yesterday and many other timing belt jobs.

What I discovered when disassembling the vehicle was galling inside the camshaft sprocket. This is not an uncommon occurrance. I've seen several repetitions of the event. It certainly is not an isolated event. The low torque value was not enough to keep the two pieces firmly together.

I have stated my case before in the TDIClub and I will say it again. If I stop one person from crashing their cylinder head, then it's worth the time to say it again and again.

DO NOT USE The 45Nm (33 ft lbs) torque setting for the camshaft sprocket bolts in either 1Z, AHU or ALH engines. USE 45 ft lbs.

No damage will occur from the additional torque. It is the setting I have always used. The 12mm bolt can certainly handle the additional load. The increased torque doen't make the camshaft sprocket any more difficult to remove and does not force it further onto the camshaft.

Some could make a case that poor workmanship could cause the sprocket to slip when using the Bentley prescribed torque setting. Grease, dirt, damage to the mating surfaces, or an out-of-adjustment torque wrench could affect the mating of the cam and sprocket and cause the cam sprocket to slip.

My feeling: when there is so much to gain and nothing to risk, the higher torque setting of 45 ft. lbs (80Nm) will do much to eliminate failures due to the sprocket slipping.

Mike_04GolfTDI · Jul 10, 2008

One question: Are they stretch-bolts?

I'm guessing probably not, but if they are, re-using them would cause this type of disaster (they wouldn't hold the right torque the second time because they would lose their elasticity).

Even if they aren't stretch bolts, perhaps re-using them is the reason they need to be a little tighter the second time. 33 ft lbs would probably be fine with new bolts, but maybe not for used ones.

Also, is there supposed to be loctite on them? (Thread locking compound)

ymz · Jul 10, 2008

Mike_04GolfTDI said:
is there supposed to be loctite on them?

No... the problem isn't that the bolts come loose... it's that the sprocket slips on the camshaft cone surface... Many here recommend cleaning the mating surfaces thoroughly before putting the sprocket back...

york77777 said:
I wonder what torque setting GURU's who do TB changes actually use?

Franko6 is as much a Guru as anyone around here... I'm sure that many folks who routinely replace timing belts have adopted his recommendations (including this one) in the past...

BTW: In a previous post Frank did say that the camshaft will break apart if you torque it far beyond the 45 Ft*Lbs figure (I can't remember the exact number... perhaps 55 or 52...)

Yuri.

Drivbiwire · Jul 11, 2008

Frank06 said:
Some could make a case that poor workmanship could cause the sprocket to slip when using the Bentley prescribed torque setting. Grease, dirt, damage to the mating surfaces, or an out-of-adjustment torque wrench could affect the mating of the cam and sprocket and cause the cam sprocket to slip.

Have the guys torque wrench tested for calibration, more than likely he leaves it set for extended periods of time resulting in INCORRECT torque values being set.

I have always used the 45Nm/33ft-lb torque value without a single failure.

I also have my digital and mechanical torque wrenches calibrated and certified annually to insure accuracy.

Frank, if you take the time to understand how a conical fit works you will see that once the cam is seated at a very low torque value that the fit is extremely reliable and results in a very secure attachment to the shaft. In fact the bolt is simply the means of installing the pulley, once pressed onto the taper the bolt does very little in respect to preventing the pulley from slipping.

What I discovered when disassembling the vehicle was galling inside the camshaft sprocket. This is not an uncommon occurrance. I've seen several repetitions of the event. It certainly is not an isolated event. The low torque value was not enough to keep the two pieces firmly together.

Improper removal of a cam pulley can result in damage to the tapered fit, this can result in failure when reinstalling the pulley on a damaged cam or a damaged pulley onto a good camshaft tapered end.

Installing a pulley onto a taper that has damage, rough edges or damage may result in slipping. It is very critical to install a pulley on to a clean(oil free), burr free taper. If there is damage or contamination to the taper increasing the torque value will not compensate for damage to the taper and could result in the pulley moving.

DB

Lug_Nut · Jul 11, 2008

These cams are hardened. My concern is the added stress on the internal threads breaking the brittle material and leaving zero bolt torque. I won't go over the spec with this bolt.
I use 33 lb*ft, with clean conical mating surfaces, and two torque wrenches, an analog beam bend (-45 to +45 lb*ft) to set the 33 value, and then a click type to check.
I don't have calibration and certification, but when fastener torque is within the range of any two of my three torque wrenches, I use both to increase the liklihood of catching if one is seriously out of adjustment.

oilhammer · Jul 11, 2008

There is also the possibility that the sprocket was not fully seated on the cone when the belt was tensioned, so that when you went to finally tighten the sprocket bolt, it was pushing the sprocket further on to the cam snout, and therefor pulling against the belt and giving an improper reading on the torque wrench.

You really have to make sure that the sprocket is all the way down on the cam, but still able to spin, before tensioning the belt, and that after you tighten the bolt, the tensioner indicator did not move. If the indicator moves, the sprocket was not seated enough before the tensioning and therefor the torque applied may be suspect, since some of that 33 pounds may have gone to pulling on the belt as the sprocket when in rather than actually on to the threads of the bolt and the seating (wedging) on to the snout of the cam.

Yet another reason why not everyone should attempt to service their own cars.

auto_tech · Jul 11, 2008

I've set my torque wrench to 35ft-lbs, allowing for differences in calibration a bit. Never had problems with slippage of pulleys, knock on wood! (not valves)

whitedog · Jul 11, 2008

lugnut said:
and then a click type to check.

OK I have a question for you. I heard about this via the internet, so I want to hear other opinions and facts.

I heard that unless the torque is less than something like 70% of the final torque, That check thing is useless. This would be because of something I have heard called "Sticktion". That is the resistance to START turning that a bolt has.

To me it sounds possible, though I'm not sure about the number, (70%) but the basic theory sounds sound.

I would appreciate any thoughts.

Ski in NC · Jul 11, 2008

I work on large marine diesels. When tightening head bolts, some are tightened to a certain ft-lb, then an additional number of degrees. Each head is marked for the degrees. I have noticed that if you pause to get another purchase on the wrench, the degrees turned in the ft-lb mode are very different than the ones where you can get a clean sweep. So yes, call it "sticktion" or whatever, torquing is certainly an inexact science.

Regarding the pulley that slipped on the cam nose, there must have been something else going on as others have posted. A taper with a retaining bolt at 33 ft-lb will lock just fine. But all must be clean and well seated.

Take a pulley and cam that is out of the head, and put the bolt in and tighten it to 33. Now take some strap wrenches or pipe wrenches and try to twist the pulley relative to the cam. If mating surfaces and threads were in good shape, I bet you cannot get it to slip.

But when I did my own tb, I went to 33 and said...hmmm, not enough. And went to about 45. A gut feel thing.

-Eric

Harvieux · Jul 11, 2008

I too, have a digital/audio torque wrench and have decided to up the torque from the specified 33 ft. lbs to 40 as a safety factor. I was keeping it close to the 33 spec for the first couple years and never had a failure but, I figured the extra 7 ft. lbs. or so would not do any harm and believe it to be a good practice at least for those vehicles going on their third TB and especially on ones that have had their TB previously done by a bafoon who torqued it to the moon (9 times out of 10 it was a dealer tech bafoon)

to where you damn near get a hernia attempting to bust the bolt loose. Know whadda mean?

Later!

oilhammer · Jul 12, 2008

LOLing at 'dealer bafoon'... seen that a lot, although usually they never bother to even remove the sprocket because they are just doing the old "Mark & Pray"

tditom · Jul 12, 2008

so, we have 2 gurus (Frank and Harv) going beyond the spec, and 3 gurus (DB, LugNut, oilhammer) sticking to the spec (after ensuring the taper is clean and burr-free).

I would vote for sticking to the VW spec, but I've only done 6 VW TB jobs

wil2007 · Jul 12, 2008

TB changeout?

I am trying to learn as much as I can while saving for a used TDI. With all the variables with these timing belts,should you plan on doing a changeout by a trusted mechanic as opposed to ruining engine? It seems like semi-cheap insurance unless you know the complete job was done by a "GURU".

tditom · Jul 12, 2008

wil2007 said:
I am trying to learn as much as I can while saving for a used TDI. With all the variables with these timing belts,should you plan on doing a changeout by a trusted mechanic as opposed to ruining engine? It seems like semi-cheap insurance unless you know the complete job was done by a "GURU".

this is absolutely the safest route

ymz · Jul 12, 2008

wil2007 said:
should you plan on doing a changeout by a trusted mechanic... unless you know the complete job was done by a "GURU".

There's the rub... some of the people named in the "trusted mechanics" thread aren't really up to the task of changing the timing belts. It's best to keep on reading here to find out who the "trusted mechanics" for this maintenance item really are... If you can get yourself to a "guru", so much the better... even if you have to drive a long distance to get there... (BTW: "guru" in this instance is someone who's known to the membership here as having done this successfully - and "by the book" - in the past and who isn't inclined to take shortcuts.)

All the best,

Yuri.

tditom · Jul 12, 2008

ymz said:
There's the rub... some of the people named in the "trusted mechanics" thread aren't really up to the task of changing the timing belts. It's best to keep on reading here to find out who the "trusted mechanics" for this maintenance item really are... If you can get yourself to a "guru", so much the better... even if you have to drive a long distance to get there... (BTW: "guru" in this instance is someone who's known to the membership here as having done this successfully - and "by the book" - in the past and who isn't inclined to take shortcuts.)

All the best,

Yuri.

IMO, they don't qualify as "trusted" mechanics unless they are honest enough to tell you that they don't have enough confidence in their ability to do the job you are asking them to. IOW, you should be able to ask anyone on that list to do the TB job, and trust their ability to do the job, or they don't belong on that list...

icecap · Jul 12, 2008

Ski in NC said:
I work on large marine diesels. When tightening head bolts, some are tightened to a certain ft-lb, then an additional number of degrees. Each head is marked for the degrees. I have noticed that if you pause to get another purchase on the wrench, the degrees turned in the ft-lb mode are very different than the ones where you can get a clean sweep. So yes, call it "sticktion" or whatever, torquing is certainly an inexact science.

Regarding the pulley that slipped on the cam nose, there must have been something else going on as others have posted. A taper with a retaining bolt at 33 ft-lb will lock just fine. But all must be clean and well seated.

Take a pulley and cam that is out of the head, and put the bolt in and tighten it to 33. Now take some strap wrenches or pipe wrenches and try to twist the pulley relative to the cam. If mating surfaces and threads were in good shape, I bet you cannot get it to slip.

But when I did my own tb, I went to 33 and said...hmmm, not enough. And went to about 45. A gut feel thing.

-Eric

Thanks for your input Ski. I also recently ran into a situation with a conical or 'morse taper' drive gear on a cummins 6C 8.3 engine in a track drill. I removed the injection pump to get it repaired since it had a crack in the governor housing. When I got the pump back from the injection shop it had a tag attached noting that the 130 ft/lb torque in the service manual should be ignored and it should be torqued to 170 ft/lb since they have been known to slip thier timing at the factory setting. Needless to say I had some reservations while I was torquing it wondering whether I was going to pull the threads but it held.

wil2007 · Jul 12, 2008

ymz said:
There's the rub... some of the people named in the "trusted mechanics" thread aren't really up to the task of changing the timing belts. It's best to keep on reading here to find out who the "trusted mechanics" for this maintenance item really are... If you can get yourself to a "guru", so much the better... even if you have to drive a long distance to get there... (BTW: "guru" in this instance is someone who's known to the membership here as having done this successfully - and "by the book" - in the past and who isn't inclined to take shortcuts.)

All the best,

Yuri.[/qote] Now I need to find the trusted "Trusted Mechanic" near the Treasure Coast, find TB changeout and add to the price of TDI.

terrydtdi · Jul 12, 2008

I have bump up the torque setting to 37ft-lbs. after hearing a few of these cases also ,for a little insurance.

Harvieux · Jul 12, 2008

oilhammer said:
LOLing at 'dealer bafoon'... seen that a lot, although usually they never bother to even remove the sprocket because they are just doing the old "Mark & Pray"

Oh yea! I almost forgot about the dealer's mark and prey white marks method which crops up it's ugly head from time to time making me wonder HTF the cam sprocket came loose so easily on those. LOL!

tditom said:
so, we have 2 gurus (Frank and Harv) going beyond the spec, and 3 gurus (DB, LugNut, oilhammer) sticking to the spec (after ensuring the taper is clean and burr-free).

I would vote for sticking to the VW spec, but I've only done 6 VW TB jobs

I have no argument against keeping it to spec. It's more of a personal freak out prevention method I decided to take without going overboard. Know whadda mean?

Later!

cheeba · Jul 12, 2008

whitedog said:
OK I have a question for you. I heard about this via the internet, so I want to hear other opinions and facts.

I heard that unless the torque is less than something like 70% of the final torque, That check thing is useless. This would be because of something I have heard called "Sticktion". That is the resistance to START turning that a bolt has.

To me it sounds possible, though I'm not sure about the number, (70%) but the basic theory sounds sound.

I would appreciate any thoughts.

I'm going to assume that sticktion is a contraction of static friction, or maybe sticky friction

Whatever you call it, there is definitely a difference between smoothly tightening a bolt without stopping and re-tightening a bolt after you have stopped turning the ratchet/wrench. This is due to static friction (in nearly all cases) being higher than the kinetic friction. Unfortunately, I don't think there's any magical number - such as 70% - since the relationship betwen static and kinetic friction changes in accordance with the force being applied. However, I'm far from a physics guru, and there could be a relatively safe guideline to use.

Franko6 · Jul 12, 2008

Ok, I expected to get a few flames... Let me put it into terms of a machinist.

The taper that the camshaft sprocket and the camshaft fit to is in much respect like a Morse taper. The morse taper is a common tooling fit for very large drill bits and tooling where extreme forces come into play. Sometimes the Morse taper fit will force on so tightly that it takes a 5 lb sledge and several whacks at the removal tool to get the taper to part. But I can tell you, I have never split a morse chuck and I've used some powerful machines pushing them.

Now, I'll bring the point of the bolt. It's a 12mm bolt. When there is no other indication, I refer to a 'standards' for torque. My standard for a 12mm bolt's torque on hardened steel is 44 ft lbs. That is a very reasonable number. Might I add, the force needed to remove a bolt at 33 or 44 ft lbs is negligible. I can't tell the difference.

But all it takes for me is a few disasters. I have already admitted, grease, accuracy of torque wrenches, dirt... there are several causes that might affect the 'correct' torque setting and the outcome.

But for me, the bottom line is this: I have a very hefty 12mm bolt and a very rugged morse style taper to pin two critical parts together. And you want me to take the risk with a torque number which I, by my personal experience have seen no less than 15 crashes because of a loose camshaft sprocket??

You do what you want. Mine is only a well thought-out suggestion based on a very sad history of the ones that have had camshaft sprockets slip and fail. I'm sure if you ask those people with the failures, they would suggest the higher number.

Just so you all don't go beserk about how damaging my idea is, I have measured how much further the camshaft is forced onto the cam by loading an additional 11 ft lbs. of torque. It amounts to .0015".

For those who use the 33 ft lbs number, don't you wonder why such a beefie bolt for such a low torque?

Something is wrong with the spec.

cheeba · Jul 12, 2008

Well said, Frank. I tend to agree with your line of thinking; and, in fact, have tightened the cam sprocket bolt to 40-45 ft-lbs. each time I've removed it.

Lug_Nut · Jul 12, 2008

quote=Franko6] ...For those who use the 33 ft lbs number, don't you wonder why such a beefie bolt for such a low torque? [/quote]Not at all.
I attribute the seemingly low torque to the:
1. hardening of the cam making the hardened internal threads more brittle, and more prone to fracture than yield.
2. taper angles increasing the expansion distortion force on the sprocket hub.
3. fine pitch on the bolt increasing the linear force of the sprocket on the cam more than a steeper pitch coarse thread might and further affecting the distortion force of #2.
Any one of these would justify lower than standard flat surface to flat surface, coarse thread bolting of mild steel. I'm not about to boost the torque I use on mine when I change it.
And it's mine only. The comment about "gurus" having the confidence to handle others work requests applies to me. I'll guide you if you wish, but you'll make the final decisions, and perform your own work.You want 45? Go for it.

Franko6 · Jul 12, 2008

Lug Nut,

I think there are a few errors in your summation.

1. The camshaft is not hardened through and through to brittleness. It's a complicated 7-step process, but what is flame hardened to a higher Rockwell standard (about 65)are the cam journals to about a .020" thickness, except at the peak of the lobe which is about .050" depth. The balance of the shaft is heat-treated for toughness, not brittleness. The bolt is not a TTY bolt. Neither are the threads. Nor are the threads brittle.

2. The amount of difference that the cam sprocket is wedged onto the camshaft is virtually inconsequential.

3. You are opening a can of worms saying that a coarse thread and a fine thread torqued to the same value have differences in applied force.

To futher diminish the argument that my higher torque value has no merit, the BEW camshaft hub, to which the camshaft sprocket is attached is applied with 100 Nm (74 ft. lbs.) and the bolt attaching the BEW camshaft hub is the same as the ALH camshaft fastener; it's 12mm x 1.5 pitch.

Why does VW now decide that the later model sprocket needs to be more than doubled? Why, when each of us who has been around long enough to see slipped camshafts, would not doubt that a 74% increase would be appropriate, when in the BEW model VW made a 224% increase and didn't even change the bolt?

Although I agree that in optimum conditions the bolt will hold the camshaft securely with 33 ft lbs, (45Nm), it's not a strap wrench I'm worried about making it come loose. It's the forces of an engine at speed. I also will not forget that in an imperfect world, adjustments must be made.

There is always room for improvement.

alphaseinor · Jul 12, 2008

I've done about 100 belts on the ALH... I'm not really a "guru" but I did stay at a holiday inn express...

I use 40 lbs, and keep some emory cloth and a can of brake cleaner (non chlor of course) for the cones.

Make it spotless and any ridges get rid of with the emory cloth then spray it again with brake cleaner and a clean rag.

The cam will break at 50 lbs, so 45 is the max I'd use on the cam.

Franko6 · Jul 13, 2008

I"ve never felt threatened for breaking a camshaft or sprocket, but I do appreciate your input. Tell you what I'll do... I've got some spare sprockets. I'll take a worn out camshaft and sprocket and I'll torture test them. When something breaks, I'll tell you what torque and where it failed. I don't think 50 ft. lbs. is the number.

One thing is for sure. The information for the timing belt job should be annotated to include check both the camshaft and sprocket for damage and cleanliness prior to connecting them.

A new camshaft comes in cosmoline. I've always cleaned that off. But I do see grease or oil being a problem.

radar912 · Jul 13, 2008

I have never done a TB change on a TDI, have researched it here in reading many posts as will have one done in the future. My question is, has anyone ever damaged a camshaft with the higher torque settings? We have plenty of examples where things have gone wrong at the lower value, anyone snap a cam to date?

joetdi · Jul 13, 2008

That bolt is to big I am sure it can handle the extra torque. Most bolts that size get a higher torque anyways.

Drivbiwire · Jul 13, 2008

radar912 said:
I have never done a TB change on a TDI, have researched it here in reading many posts as will have one done in the future. My question is, has anyone ever damaged a camshaft with the higher torque settings? We have plenty of examples where things have gone wrong at the lower value, anyone snap a cam to date?

I have personally seen the results of overtorqued cam pulley bolts. The bolt (grossly overtorqued) resulted in the cam shattering between the taper and #1 cam bearing when the pulley was removed on a subsequent timing belt (1Z motor).

The older procedure used a drift and hammer to knock the pulley off the taper. Since the pulley had been pressed onto the taper by way of over torqueing the bolt, the pulley caused enough offset force to crack the cam when it was removed with the drift/hammer procedure due to the higher force to remove the pulley from the cam.

Had the cam been torqued to spec, the pulley would have released off the taper and not broken the cam.

The other concern is deforming the pulley's female taper by way of overtorqueing the bolt. I would agree that a few ft-lbs of variation (+X/-0) may not result in damage, however just adding what you may "think" is ok may in fact result in deformation and damage to the taper resulting in a failure to maintain a proper mating to the cam's end taper.

The issue with the cam bolt is not the bolt! The issue is the angle that the taper is set at is the limiting factor. Increasing the bolt torque applies significantly higher force in respect to that generated by the two tapered surfaces coming together. The VW spec is not predicated on the limits of the bolt but the forces generated by the two contact surfaces of the taper.

The BEW uses a different angle thus a different torque spec is utilized. Additionaly, the PD uses a keyed cam to pulley so timing is maintained and assured via this method. The external pulley accounts for variation between belt production lots and uses three elongated bolt holes to adjust for any variation between timing belts. Also the BEW has to endure much higher rotational vibrations and forces due to the increased loads on the valvetrain by way of the PD system

Comparing the two is an apples to watermelons comparison and not valid.

The best advice is to stick with the VW spec!

DB

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