Full Build CR - Piston to head clearance

I'm wondering if I can get some opinions this weekend on my CR engine rebuild. I'd like to know what you think about where I'm at with piston to head clearance and what the implications will be. Engine is at a local reputable engine shop that does a lot of diesel work.

Quick summary of the build:
-IE Tuscan Rods
-Fresh bearings and rings all around
-Main and head studs
-Machined pistons for slightly lower compression ratio
-Port and polished head
-Valve job
-Upgraded valve springs
-Colt stage 2 cams
-GTB2260vklr turbo
-Darkside intercooler/piping
-CP3 Pump w/Bobby Singh injectors

So the engine shop reassembled the short block and we had 1.27mm piston protrusion. With a 3 hole head gasket (1.71mm thick) that left me with .44mm piston to head clearance. Tight according to the engine shop. We spoke on the phone and agreed to machine the pistons and skim a bit off to allow for more clearance.

In the meantime, with some help from top members in this group (TDISyncro, MacRadiators - thanks guys), I've come to learn that the spec I'm looking for is .5mm-.6mm clearance.

Then I found out the shop took off .635mm. This leaves me with a new piston protrusion of .635mm. With a one hole gasket (1.55mm thick) I'd have .915mm clearance. Puts me .315mm fat on the clearance.

I'm trying to decide if I should have the shop deck the block back down to get me within that .5mm-.6mm spec. At this time the car will see a half dozen or so track days a year and is not really a daily driver. Maybe compound turbos someday.... What will that extra head space do? Probably more smoke? Lower compression ratio too much? Was shooting for about 15.5:1.

Roll with it or have them disassemble the block and deck it down to get in spec?

All opinions appreciated.

I'd just roll with it as is. You may want an even bigger turbo at some point, and the slightly lower compression ratio will work out nicely.

Now you need a real turbo to boost it.
A well set 2263 or 2565 will do just fine

Is even a thinner head gasket available than a one hole somewhere?

Damn, I wish I knew exactly where I am at with compression ratio at this clearance.

Not sure about the gasket, but you can do a compression test for the ratio. Just divide the test psi number by the barometric pressure in psi (usually around 14.7). Much easier than crunching all the dimension numbers.

With 0.9 mm clearance you can expect poor cold starting and lot of white smoke. 0.5 mm is all you need.

I think that should be pretty close to your 15.5:1 goal with the one hole gasket. It may not be the ideal area to gain volume for squish though. I guess I'd be afraid that they would get carried away with the milling on the block, and it would never end.

Yeah. The other thing I didn't explain well above is that the piston bowls were enlarged slightly taking the lip out. So now they are de-lipped and skimmed.

I'm going to have the block decked.

We can measure exactly how much to take off the block to hit .55mm protrusion.

Read this...been together for over 100k miles with 0.4mm piston/head clearance and I'm not nice to it...

http://forums.tdiclub.com/showpost.php?p=3682956&postcount=225

You probably could have left it alone, but looks like we're a little late for that .

It'll "work" in any of the scenarios posed above, but with the resultant consequences (hard start cold, white smoke, stinky idle, etc). Ideally you want minimal volume between the piston and head so that all of the air is in the bowl rather than trapped in the doughnut shaped disc between the piston and the head.

If you don't mill the block, you'll be down quite a bit in compression ratio - roughly 0.1 points of CR for each 0.001" (or 0.025mm) so you'll be down ~2 full points just due to the piston protrusion (or lack thereof). Not sure on the de-lipping effect on CR. Valve protrusion is also a significant variable in CR - very easy to alter the CR by a few tenths just by where you set the valves when machining the seats.

I'd suggest they take the same 0.635mm off the block - just make sure that the timing belt tensioner can take up the difference.

Thanks, great info

travis45 said:

Is even a thinner head gasket available than a one hole somewhere?

Damn, I wish I knew exactly where I am at with compression ratio at this clearance.

Click to expand...

the alh-era gaskets were 3 piece for 1 hole (two wave plates and one thick flat), they added a thin shim for 2 hole, and 2 shims for 3 hole

whereas later gaskets seem to be 3 equal thickness wave plates for 1 hole, 4 for 2, and 5 for 3
...so you should be able to drill the rivets and remove one of the plates for a "zero hole" equivalent gasket

ETA: bringing belleville washer thoughts into this mess, one would think the thicker gaskets of this later type would have more spring force to them than thinner. Probably of no actual consequence but still a thought to have.

That is a great idea about the head gasket. But here is a picture of a two hole. You can see two layers on one side, and one layer on the other. I have to assume the one hole has one thing layer on each side. So I don't know if you could peel an extra layer off the one hole? Could I just use the middle layer?

Fix_Until_Broke said:

Read this...been together for over 100k miles with 0.4mm piston/head clearance and I'm not nice to it...
It'll "work" in any of the scenarios posed above, but with the resultant consequences (hard start cold, white smoke, stinky idle, etc). Ideally you want minimal volume between the piston and head so that all of the air is in the bowl rather than trapped in the doughnut shaped disc between the piston and the head.

Click to expand...

So the doughnut is less surface area on the CR compared to the ALH piston. So is your number of one tenth of a point on CR for an ALH piston or a CR piston?
All great information and really appreciated. Really need this to be right.

travis45 said:

So the doughnut is less surface area on the CR compared to the ALH piston. So is your number of one tenth of a point on CR for an ALH piston or a CR piston?
All great information and really appreciated. Really need this to be right.

Click to expand...

Arrgghh - just deleted my response accidentally ...

Good catch - my rule of thumb is based on the whole piston protrusion, so just a simple disc volume, however your situation is just the annular area outside the bowl which won't be as much.

For some bar napkin engineering, you could just calculate the annular area divided by the full piston area and if this number is (for example) 0.7, then the CR would change by 0.07CR per 0.025mm.

The 0.1CR/0.001" is based on ALH geometry and this value changes with bore, stroke, starting CR, etc. My apologies for possibly leading you in the wrong direction. If you can share what your engine's OEM bore, stroke, CR, valve diameter, valve protrusion from the cylinder head are, I can see if I can find my spreadsheet and plug the numbers in to get a better guess as to where your CR is or will be. If you have some bowl dimensions both pre and post lip would be helpful as well so we can better guess what that effect is on the CR.

[EDIT] - Found some info on the CR model - See post below

http://forums.tdiclub.com/showpost.php?p=3731487&postcount=306

[/EDIT]

travis45 said:

We can measure exactly how much to take off the block to hit .55mm protrusion.

Click to expand...

That's probably the way to go then. I'd set it up for the two hole gasket so you can do an easy adjustment in either direction by switching to either of the other gaskets.

Oh man... This is what TDISyncro has been telling me all along. Do the math and figure CR out before I go machining the block down further and redefining the term "short block." Hahaha.

Need to just time out, and settle down. My anxiousness to get my car going again almost cost me more troubles.

Going to get the measurements and find out CR before deciding what to do next.
The other thing worth considering is the injection height. If my nozzle is 1/3mm higher, will that have an impact on things?

Thanks again for the help.

Travis

turbobrick240 said:

That's probably the way to go then. I'd set it up for the two hole gasket so you can do an easy adjustment in either direction by switching to either of the other gaskets.

Click to expand...

Yeah, that's what I asked them to do in the first place. There was some miscommunication or a mistake made.

travis45 said:

Oh man... This is what TDISyncro has been telling me all along. Do the math and figure CR out before I go machining the block down further and redefining the term "short block." Hahaha.
Need to just time out, and settle down. My anxiousness to get my car going again almost cost me more troubles.
Going to get the measurements and find out CR before deciding what to do next.
The other thing worth considering is the injection height. If my nozzle is 1/3mm higher, will that have an impact on things?
Thanks again for the help.
Travis

Click to expand...

Good call to step back and re-evaluate.

I found the spreadsheet, so whenever you get the info you can post it here or email it to me at fixuntilbroke at gmail dot com. I can send you the spreadsheet if you want as well - it's pretty straight forward.

Redefined short block - I like that

With the pistons shaved and the block decked, won't the nozzle be deeper in the bowl compared to stock? I'm not familiar with how the CR injectors are mounted/sealed in the head, but maybe you could use a thicker copper washer to pull it back out a little? Need to check that the spray pattern doesn't start hitting the bottom of the cylinder head I suppose?

travis45 said:

That is a great idea about the head gasket. But here is a picture of a two hole. You can see two layers on one side, and one layer on the other. I have to assume the one hole has one thing layer on each side. So I don't know if you could peel an extra layer off the one hole? Could I just use the middle layer?

Click to expand...

maybe it is the PD gaskets that are multiple layers of the same thickness, or just one brand of aftermarket gaskets...

I know the mahle one in my ALH at the moment is of that construction, probably wouldn't fit an 81mm CR piston through it through

I think the thinnest is the way to go if you can. Less layers to leak.

The engine shop promised me an exact compression ratio calculation. Hopefully will have that tomorrow.

Well, just got a call from the engine shop. They ran the cr calcs through several different calculators and came up with 16:1 with a 1.55mm head gasket.

Seems about right. Lowered from stock cr, but not too much. I am relieved.

Thanks everyone for your help.

Got my short block from the shop. They are waiting on valve seals for the head, but I grabbed a picture while I was there. Looking forward to start assembling. Pistons de-lipped and port and polish complete.

[/IMG]

[/IMG]

Cant see pics. Seems to be broken?


Sent from my iPhone using Tapatalk

Try again, was messing around with the links.