Measring Cylinder Bore & Piston Protrusion Measuring Questions

Measuring Cylinder Bore & Piston Protrusion Measuring Questions

Ok I'm not sure of the best location for this thread but I know most of you hardcore modded TDIs have had to cover this at some point.

What tools are you using to measure the cylinder bore diameter? I just tried the cheap telescoping gauge set (also called snap gauges) from Harbor freight.



I plan on getting a little more precise tool from summit racing since the harbor freight tool was a pita to use:



For the Piston Protrusion I figured out how to get a measurement but it took more time then I thought. Using a magnetic base I was thinking I could sweep the dial indicator out and get a measurement but because of all the angles I figured that it was accurate that way. I ended up getting my measurement in one spot and then released the magnet and repositioned the dial and stand without moving any of the joints. This worked well. I also learned that the point directly above the wrist pin was the best spot for the peak measurement. Is this similar to others' experiences? Any tips?



This is the first time I've had to do this and I'd like to do it right. I'm open to any tips or suggestions on tools and technique. Thanks

I use basically the same procedure for measuring protrusion. I do however check four spots on the piston when doing it just to make sure I average out any cocking of the piston in the bore...or a bent rod.

For the bore - I took a new original sized ring and measured the gap at several points. Gap in spec should mean the bore is correct. I tried to borrow a bore gauge but couldn't track one down.

For protrusion, I used feeler gauges laying on the block next to the piston. Only needed to confirm that they were in the range of the 2 hole gasket that originally came with the car.

I just eye-ball it.

as been siad, put something straight on the bore ith pistons down, put in the middle of this straigt piece a dial gauge straight down, then turn engine to tdc and read protrusion

Welcome to the world of experience.

After a lot of time using the rod guages for bore measurements it starts to make sense. The unworn top of the bore (if you have not honed it away) should measure differently than the top of the bore where the piston ring has worn the cylinder, and the bottom unworn part of the bore will measure differently. It is just over and over and over till you get the hang of it. A trip to a garage and use your tools on a really worn block will help get the hang of it.
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The piston protrusion can be a trip. First off you are looking for bent rod, or a bent and twisted rod. There is also the time you can seem to find a shorter rod (two bends and maybe a bow). You are looking for a situation (usually in one cylinder where things do not make sense). Pistons can be forced to move around in the bore some. I like to level them from side to side of the block first, then move to the end to end movement (2nd measurement over piston pin ends). It should very soon appear when you do all this, that things keep on changing as you go from one measurement to another. You have to recheck things over and over. If one piston on one end of the piston pin (toward one end of the block) tends to be high and the other end of the piston over the other end of the piston pin tends to be low you are probably looking at a bent rod. And then the real thrill is when you have a bent and twisted rod when nothing makes sense. Sometimes you can seem to force the piston measurements to be correct, but just lightly touching / pushing one area over a piston pin end gives the high low measurement.


If nothing is wrong with the rods the piston protrusion can be simple and fairly quick. If a rod is bent and twisted your brain can go tilt fairly quickly. It is about experience. When you get used to it you do all this stuff without even thinking. You have a finger on each end of the part of the piston over the piston pin ends, rock things a few times take some measurements and let er roll on to the next cylinder.
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The real thrill of life is when you do an air cooled VW and the cylinders are moving in the midst of trying to hunt for a bent rod. The air cooled VW valve drop almost always broke the piston. Just take that rod out put it with another good rod (on a pin) and check the big end to see if the faces were parallel. Then flip one rod and recheck...........I have taken rods to have them straightened and got them back still bent some (checked by putting them on a pin).

lol
eddif