Cylinder height problems? A solution.

jpnovak · 09-22-2008, 12:30 PM

I am building a 3.2SS after my 3.0 decided it wanted to live with a few too few head studs. I knew this was an eventual problem and so I bought some 98mm Mahle PCs several years ago. Of course, I was less knowledgeable then.

the PC set has had a few issues. First the pistons had had the rings cut one size larger sometime during their life. The skirts measured out fine it just took a few phone calls before Total seal could make the correct size ring.

The cylinder spec out fine for bore and ovality. It was not until the other night that I actually measured the height from the spigot seat to the sealing surface. HMM 4 are identical and two are identical but a different number than the first 4. Crap. Two mismatched cylinders. The difference was approximately 0.30 mm. Way too much for a single bank to have. I did not feel comfortable shimming the difference. So I called on a good friend who is an amazing machinist. I explained my predicament and he said to meet him at the shop on Sunday. He was working on his car that day.

After some measuring and discussion of the problem we decided on a solution. I think the pictures speak for themselves.

Hmm no pictures upload button. Let's try again in a second.

jpnovak · 09-22-2008, 12:33 PM

The cylinders were all jigged onto the CNC bed. Ben locally machined 6 plastic plugs for the cylinders to locate. Then we marked each cylinder with a sharpie and ran concentric circles on each cylinder until the all matched. We were taking passes at Z = -0.005mm

Now they are all within 0.01mm in height.

Notice we did not cut on the upper sealing surface so not to chip the nikasil.

Thanks to Ben for all his help. I worked on his 928 while he was milling parts.

911 tweaks · 09-22-2008, 02:31 PM

Looks great Jamie... just make sure to measure the amount of the base of the cylinder that goes into the case... make sure it is not too long i.e. sticks too deep into the case and possibly affect windage and/or come into contact with some moving part(s)... maybe a good time to moon the cyl's base, though most feel it does very minimal of help... I would do it on a special build if it were me but I always consider or do the extreme and weird stuff :-)
Best of luck... sounds like a great build
Bob

tom1394racing · 09-22-2008, 03:55 PM

Jamie

Be sure to check deck ht and compression.

I have seen some used cylinders that have been cut well below min ht group specs to raise compression.

Good luck and keep us posted.

dtw · 09-22-2008, 04:14 PM

Hey man -
Is this getting you closer to 1.0mm deck height all around? That 1.4mm just sounded too high to me when we talked on Saturday - especially for this configuration.

Are you going to CC again or are you comfortable enough to change your height measurements and recalculate CR?

Nice catch, by the way. You'd have probably figured it out though, when your cam wouldn't turn on one side...

jpnovak · 09-22-2008, 08:00 PM

Bob, The cylinders have plenty of clearance at the case webs. We just cut the tall ones to match the short ones. Even better was that we trued up the sealing surface on the short ones. They were ever so slightly warped. It was really cool watching the sharpie indicating line become nearly transparent as we milled off so little.

We did talk about mooning the cylinders. I decided against it as this will only be a 7000 rpm motor, cam limited power. I won't be spending that much time sustained on the rev limiter. Were I spinning it faster I would have mooned the cylinders. At least they can't moon me back.

Tom and Dave, Prior to this machine work I was indeed getting 1.4mm deck height. When I put them back on the case last night I measured 1.08mm. This was taken with no base gasket. Using this number I recalculate a CR = 9.94:1. I was aiming for 10:1, right on target. I do not plan to CC again. The measurements were taken off the engine and I trust they have not changed.

Wayne recommends 1.25 - 1.5mm in his book for a stock build. Bruce Anderson recommends 0.030-0.040" (0.73- 0.98 mm converted). I feel safe running at ~1.1mm with no base gaskets.

I admit that when I torqued down the cam towers and heads on the right bank last night I was relieved to find that the cam spins effortlessly. Well, with the same minimal drag that the bare cam tower gives. These cylinders and heads are nealy dead flat. my 0.0015" feeler gauge will not slide under a machinists rule across the heads or cylinder tops. That is as thin as I own.