Deck height variance in pistons?!
 

I installed my Mahle pistons and cylinders # 1 and #4 on the engine tonight and noticed that from top to bottom or from top to bottom there was a deck height variance by as much as .007" on them.

Is this normal?

Thanks,

Did you replane or measure your spigots as detailed in Chapter 3? I thought this was an AL case - shouldn't have too much noticeable variance.

How are you making this measurement? With some pistons, it's difficult to get an accurate measurement.

This could be cause for some concern - please give me more details...

-Wayne

Wayne,

THey were re-paned the last time the motor was built, and that was only 2000 miles ago. I am measuring it at the very top of the piston. They have a flat edge at the outer edge of the piston about 1.5mm wide, but I am also concerened with getting a consistent and accurate measurement which is why I'm thinking the "solder" method will be necessary...

THe variance is only one the #1 cylinder, not the #4, so it's probably not the cases being mis-aligned (sealed in a "V" shape) otherwise I would likely see this on both sides.

I'm going to try a few other cylinders today and see what I come up with...

Is the piston rocking and giving different heights?
-Chris

Chris.

YEP! On a hunch I went back in and rocked the pistons slightly and the will vary about .005" if you move push on the top or the bottom slightly. If you are VERY careful you can get both top and bottom to move the same amount.

Once I played with that a bit I got measurements in deck heights that varied on a side by no more than .001" - .002" Is this aceptable?

So now I can run .5mm shims on one side and .25 on the other side and get a deck height of 1.25mm all around... THen again I could go up to .5mm and .75mm and get heights of 1.5mm all around. I wonder how much this affects compression? Wayne?

I'm not making final decisions untill I can mount the heads and do the platicene or solder guauge measurement.

Being is that this is a twin-plugged race motor, I want some extra compression, but why push it?

If the case was done properly, you shouldn't have any variation among the three. Take the cylinders off, and recheck the cylinder planes - sounds like they may have not been done properly. You need to make sure of this prior to doing your deck height, otherwise you may have off-measurements.

Wayne,

How do I check this?

Chris,

To check that the cylinder bases are all in the same plane you need a good (machinists) straight edge. You lay it across the the three cylinder bases and see if it rocks, or if you can see light under where it bears on the case. If there is variation a machinist can mill the case and true it up.

Not what you want to hear after youve come so far.

Better safe than sorry though... I expect it's probably my foulup in measurement... I don't have a quality tool.

I'm not sure why 1 or 2 thousandths are significant enough to warrant disassembling the case (Since it ran fine this way for years), but I will look into it.

What is the spec for deck height? were talking several thousandths of an inch here. I would find out what is acceptable before I undid or revisited work you have already done.

SOund advice. I haven't found it yet, but I haven't looked that hard either. I certainly will find out what the allowable variance here is before I start to worry...

There is no documentation in the technical manuals about deck height at all... nor in BA's book. Hmmm

Well I'm digging around in the factory manuals and have noticed that acceptable rod variance is .002" between centers... THis being the case, one could expect to see variance in the deck height by this much, in fact if ajoining rods where + and - .002" (.05mm) you could see variance up to .004" (.10mm)

They also list acceptable cylinder height groups. Each group lists acceptable standard cylinder install height as a variance of .025mm (.00098" or .001" rounded) meaning the cylinders within a group can also vary by a thousandth... Add this in to the rod measurement and you see a total DECK HEIGHT variance of .0025" on a single cylinder and .005" (!!!) between adjoining cylinders.

Since the factory only offers cylinder shims in incremental sizes of .25mm that do not account for variance in the rod dimensions this MUST then be acceptable.

What would concern me is if the variance was due to varying cylinders (not likely) or varying case dimensions, so I should not only check the case spigots, but also the cylinder heights, those two would be the show stoppers!

I checked the cylinder heights with a guauge and the case spigots with a borrowed machinists straight edge, they all are consistent. THe spigot surfaces are perfect, and the cylinders are all with .001"

So....

THe variations must be due to allowable runout within the pistons, rods, and crankshaft dimensions... Again, they are all within .002" of each other...

I will check true piston/head clearances later.

Wow Chris! Looks like you have been busy. I never would have thought it would be so tough...better your motor than mine ;)

Just kidding. I hope you get it all worked out to your satisfaction. I commend you on your attention to detail. If I didn't know better I'd thought you were an enginerd.

Later.

Well a true engineer would figure out why I can't get the same exact readings twice. Then again my new caliper (yep) goes down to .0001" accuracy, so perhaps I'm being a TAD picky...

I've got a whole section in Chapter five on setting the deck height...

-Wayne

Ah ha! New calipers, that's why you have been so busy, been playing with the new toy. Repeatability by human hands on the level you're looking for is probably gonna be tough given the conditions (movement of piston and consistency of gauge placement. But it looks like you're giving it a good fight. Ahh, I like this sitting back and learning from others thing ;) Good luck.

My straight edge is a piece of ground "stock". It is too long to be able to measure the spigot flatness directly so I install the cylinders without any gaskets and (carefully) place my straight edge across the tops of the cylinders. My 3.2 case was not flat so even aluminum cases are not immune.
-Chris

First, are you measuring this with the cylinders torqued down with the tools they sell or with sockets over the studs to simulate the heads when torqued? Do this without the base gaskets in so you don't crush them before final assembly.
I would then rock the pistons one way and measure, then rock them back the other way and take the average of the 2 readings. Or just make sure to measure above the wrist pin axis since this takes out any effect of the pistons rocking.

This sounds like a non-issue to me. If the spigots are level, the cylinders are all the same height, and the rods are correctly machined, then you should have the same deck height across the board...

-Wayne

Chris quit analyzing and bolt it up!!

Har har.. That's what my shop said too Tim.. :)

I am a little out of place here on this board as I am not building a flat six right now, but in this circumstance, I believe a method to deal with the problem mentioned is to move parts around the engine until you have the optimum set up. On a 4-cyl motor I built, I had a similar problem and combined parts until they matched up. I mocked up the motor 3-4 times until I finally had to just work on one cly to get it the same as the other three. After all was decked correctly, I sent it all out for balancing. People have been mating the long rod with the high wrist pin for a long time before us.

Zeke,
Thats the thought I had too works on any engine!


Keith