Piston to deck height

[Dukes2fast]

I just measured my piston to deck height at TDC and found out it seems to vary quite a bit from cylinder to cylinder and each cylinder varies from side to side. 68 440, w/stock 906 heads. New pistons, KB236's. Crank turned, rods conditioned. Crank, rods and pistons balanced. The deck was cleaned up removing .010 and checks out. Measurements vary from .004 to .015 below the deck at TDC and each piston varies about .005 from one side to the other. Is this normal? What is causing this and is it a concern? Just trying to get this rebuild done right. Thanks for your help.

[perfmachst]

hello, the variations can be the block to crank dimensions, the center to center dimensions of the rods, the strokes of the throws,piston comp. distance. all these can add up to what you have found. to blueprint a motor, the block has to be square decked. all four corners will be excatly the same from main bore to deck. rods will be sized to same C.C distance, the crank will be indexed, strokes equalized. pistons will have all same comp. distance.
with all this done, the piston to deck heights will be all the same. this will make a lot more power also. just food for thought.

[Dukes2fast]

I have gone too far to start over again. So, I guess I will have to live with it the way it is. Are these measurements real bad or are they within acceptable tolerances for a street/strip motor?

[John Kunkel]

If the block deck height wasn't equalized when it was surfaced, there could be a variance there and if the rods were rebuilt with no concern for the center distance between the large and small ends, it would explain the wide variety of dimensions.

One way to equalize the discrepencies is to measure the volume of each combustion chamber and match the tallest piston/rod combos with the largest chambers.

[benno318]

use one rod and piston combo to check all of the bores to verify if it is the deck out of whack or the centre to centre distance of the rods themselves. the centre distance of the rods can be checked without pistons assembled simply using vernier calipers. not how a machinist would do it but good enough for us for an indication if they are different.
often if a below average deck job is done, the measurement can taper back or forward, gaining or losing clearance as you go along,
(eg. front cyl may have .010", #3 has .013, #5 has .016, #7 has .021.)
if this is the case it would be relatively easy to have a good machinist square up the decks for your peace of mind. it is not easy to find machine shops that care as much about your stuff as you do, and when you find one you can trust, stay with them for life!
(where does perf.machst. work?)
ben simpson

[cutting torch]

Finding a machine shop that will explain the how and why of everything they do to your stuff is a good indicator of quality work. Get educated, and talk to the machinist that will be doing the work. If he will talk to you, you have won half the battle. If you got educated, you will know if he is blowing smoke!

torch

[Dukes2fast]

I have taken this block back apart twice now after finding mistakes the shop made. I really would like to get it running. If I run it the way it is now, with these variances in the piston to deck height, will I see a significant drop in performance?..I mean, other than not performing up to it's potential, can it cause any damage to run it like this? Thanks again for all your help.

[cutting torch]

It won't hurt anything. Where on the piston are you measuring? Depending on where you measure, piston rock can effect your measurements.

torch

[Dukes2fast]

I tried to hold down on the piston to eliminate piston rock. I measured each piston at three points. The bottom centered, and at the center on both sides just below the valve reliefs. The differences don't seem to "run off" from front to back or vice versa, that makes me think it is differences in the rods, since the block was supposed to be squared and appears to be flat by my measurements.

[wedgehead]

I would think that your problem is probably in your rods. Were the rods bushed, using bronze bushings in the small ends? If so the center to center length is likely off by a few thousandths. I would take the pistons and rods to a competent machinist and make sure they are machined within .001 of each other. If they check out then crank may need indexing again. Throws may be off a bit. Good Luck, Wedgehead

[pishta]

You could always cc the heads to make up for any botch in the piston height, if the motor just isnt going to be taken down again. even a sunken valve can mess up a perfect balance.