Thread: piston deck height

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Below is a picture of the setup I used to check piston deck height. First I zeroed the gauges against the deck then swung them over the edge of the pistons and rocked (no rings) the pistons until both gauges measured the same. Assuming that technique is ok, and please straighten me out if not, the results don't look good to me and I'm not sure what to do about it.

I have a new 383 rotating assembly kit from Speedomotive - Scat crank, Scat 6" rods, KB forged pistons. The block was decked to zero according to the person I bought it from. Don't have anything to verify so I tried using a carpenter's square so I realize I could be off there. I did verify compression height of one piston at 1.1305" (KB says 1.13). Haven't checked crank stroke (no appropriate tool) or rod length (rods/pistons are floating pin and I don't want to remove the spiral locks unless absolutely necessary). I did check the deck with a 24" machinist's straight edge, diagonally each way and then lengthwise down the middle. Found only one spot where I could slide .003 feeler gauge underneath.

All the pistons are above deck. I expected them to be .005 above deck but that doesn't seem to be the case. This will be a street motor. Any comments or advice please.
1 .007
3 .009
5 .017
7 .015
2 .021
4 .015
6 .019
8 .012

Originally Posted by GMR

Below is a picture of the setup I used to check piston deck height. First I zeroed the gauges against the deck then swung them over the edge of the pistons and rocked (no rings) the pistons until both gauges measured the same. Assuming that technique is ok, and please straighten me out if not, the results don't look good to me and I'm not sure what to do about it.

I have a new 383 rotating assembly kit from Speedomotive - Scat crank, Scat 6" rods, KB forged pistons. The block was decked to zero according to the person I bought it from. Don't have anything to verify so I tried using a carpenter's square so I realize I could be off there. I did verify compression height of one piston at 1.1305" (KB says 1.13). Haven't checked crank stroke (no appropriate tool) or rod length (rods/pistons are floating pin and I don't want to remove the spiral locks unless absolutely necessary). I did check the deck with a 24" machinist's straight edge, diagonally each way and then lengthwise down the middle. Found only one spot where I could slide .003 feeler gauge underneath.

All the pistons are above deck. I expected them to be .005 above deck but that doesn't seem to be the case. This will be a street motor. Any comments or advice please.
1 .007
3 .009
5 .017
7 .015
2 .021
4 .015
6 .019
8 .012

Are you forgetting about your compresed gasket thickness??

I guess I am un-sure what he really wants to know.

well your blocks looks to be out of square from #1.007 to #7.015 there is .008 and #2 is .021 and #8 .012 so it 013 so the block has the decks running down hill and up this is very common i use the bhj tru-deck with helps keep every thing square and sets every thing of the cam and mains to keep the deck square i check the blocks for twist with a big mic from main saddle to deck this tells me were the deck is in the 4 corners of the block if deck off mains the deck should be square to the mains

You might tru switching piston/rod assemblys between the high and low holes---

I agree with Pat that the Chev blocks seem to run uphill on one side and downhill on the other---if they are just decked from the original surface this will remain. We deck from the mains and on Chev you can bet that it will take most of .020 to sguare things up. Now that you have it done about your only choice is to swap holes with the pistons to see if any differences will help out.

well with a flat top go from the center of the pistons to read them like denny said to get the max reading on the gage with no rings rock piston and put a parallel across the top and slide a feeler gage on the low side of the piston this will be divided by two this will be were your piston is in the block

Originally Posted by jerry clayton

I agree with Pat that the Chev blocks seem to run uphill on one side and downhill on the other---if they are just decked from the original surface this will remain. We deck from the mains and on Chev you can bet that it will take most of .020 to sguare things up. Now that you have it done about your only choice is to swap holes with the pistons to see if any differences will help out.

i did a chevy it was so bad i took every thing off my mill and check the mill for runout the block decks from side to side were off from 90 and up hill

I guess I was thinking it was they way he was checking things.
I would try corner to corner with the straight edge.
If you can get more than .002 in there than I would be decking it.

As Jerry suggested, swap 1,5 and 2,8 and see what you get. And like Denny said, place the dial indicators at 3 O'Clock and 9 O'Clock as you stand at the side of the motor. A difference between the 3 and 9 positions could indicate a bent rod or one that was not machined correctly. The pistons are usually pretty square.

When all is said and done, choose a gasket thickness that when added to the pop-up of the tallest stack, will give you a squish of 0.045" to 0.050". I use 0.035" to 0.040" for a tight fitting cast or hypereutectic, but forged pistons use a little looser fit and have more room to swivel on the pin, so a little more clearance on the squish is called for. At least that's the way I do it. There are others on this board who have a lot more engine building experience than I do and may suggest something different to you.

If you want to know the length difference between all the piston/rod assemblies, just mount each of them in the same hole and dial indicate them.

like i said if new rods crank rods my money is on the block. i deck them all the time . i am resting up today have 4 to deck monday the machine is very slow but i get a very good finsh and my rottler f2b boring bar in the back ground i love this machine

Originally Posted by DennyW

Dang Pat, you make me want to make a trip up there, and have you do my other engine block. You never know, I might get lucky and win some money...

well you do know i would take good care of you

Thanks all for the great replies. I took pictures of each of your suggestions as I understand them. Please let me know what you think.

I did swap 1 and 5 pistons, as Jerry suggested (can see that in pictures), and results were very similar to original.

Techinspector1, do you think I'm ok or does the difference with the indicators at 3 o'clock and 9 o'clock indicate a problem as you mentioned?

I have a description with each picture which I hope is helpful.

Are you saying that after the 1-5 swap, you still have 0.007" on #1 and 0.017" on #5??? If so, then the stacks on 1 and 5 are identical and the rod journals are ground differently from the centerline of the crank.

Did you find more than a few thousandths difference between the 3 and 9 positions on the pistons?

What's the thickness of the gauge in the 600 photo?

[QUOTE=GMR]Thanks all for the great replies. I took pictures of each of your suggestions as I understand them. Please let me know what you think.

I did swap 1 and 5 pistons, as Jerry suggested (can see that in pictures), and results were very similar to original.

Techinspector1, do you think I'm ok or does the difference with the indicators at 3 o'clock and 9 o'clock indicate a problem as you mentioned? [QUOTE]
not one thing is going to change it the deck is going up and down hill with new rods and new pistons should not be much more then 003 max from any rod piston going to any hole front or back your deck is not square to the main housing bore center line you have a $hitty DECK JOB the line bore can be any were on the block side to side up or down when you deck to the center line of the main housing bore your deck is now square to your crank mains