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Old 04-08-2012, 09:56 PM
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Did the machine shop thrash my block?

This is my first engine build up (SBC 350). Perhaps I was expecting more for my money. I told the machinist to deck it .020" which should leave me .005" deck clearance. What I got was not even close and all cylinders vary quite a bit! See the attached image for my measurements. My goal is to have a nice tight squish and these numbers are screwing with my plans.

Pistons are H345ACP60 which have the standard compression height (1.560").
The rods are factory, used and reconditioned.
The crank is factory, used, and reground.

I understand that there is rod stretch, and after reconditioning the rods and regrinding the crank there may be some variances in rotating height, but I think what I am seeing here is crazy. I am not a classically trained machinist. I used a bolt down dial indicator to bring the pistons to exact TDC. Then used a straight edge on the block and slipped some feeler gauges between the edge and piston (front and rear of the piston, parallel to the wrist pin to factor out piston rock). Yeah there maybe a +/-.002" at the most in my measurements, but I doubt it.

1) Are these pistons known to be a little funky in the exact compression height as well as a nice level wrist pin hole?
2) Are these numbers normal for a "regular" machine shop (I am not building a Swiss watch or F1 engine)?
3) Is the difference between and left and right side due to core shift? Should I use a thinner gasket on the left side to compensate?
4) How could he have taken off .020" and the pistons be so far down the hole (advertised piston compression height not accurate)?
6) Should I just use the average numbers to figure out my compression ratio and squish?
7) For you guys that zero deck your block, do all your pistons come to exactly .000" clearance?
8) Any advice on what to do?
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Old 04-08-2012, 10:13 PM
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You did something in your measurements wrong as there is no way you can get that much variance between cylinders. Example, if hole 1 is .004 down the bore and 3 is .009 down in the hole then five can't be back down to .004, that would make the deck look like a wave.
Decks are often tipped, only tip is measured front to back or back to front.
Example is lets say cylinder 1 is .002 down in the hole and cylinder 7 is .020 down in the hole, that is called tip.
Do your measurements over.
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Old 04-09-2012, 05:28 AM
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Rod length after re-con can also be a factor. I have seen re-con rods that were purchased from a well known source that varied well over .020".

Modern pistons are done on CNC machines and are usually Gnats ***. Rods are hand done and each is different.
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Old 04-09-2012, 05:55 AM
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When we do a block we lightly cut the decks to square the decks to the crank and cam centerlines. Then the block is bored and honed. We then mock up the rotating assembly in the block to get a true deck height and cut accordingly.

In your case you told the machinist to cut .020 off the decks. If the decks were different heights to begin with then they're still going to be different heights. I wouldn't get hung up on the edge dimensions. Taking the averages I would say the readings on each bank aren't all that far off. You're correct that one bank is .004 higher than the other.

You have three choices: Take the rotating assembly out and get the decks cut accordingly, run two different thickness head gaskets, pick a gasket thickness that's a safe compromise. In any case the block is not junk.
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Old 04-09-2012, 05:56 AM
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Quote:
Originally Posted by Silver Surfer
This is my first engine build up (SBC 350). Perhaps I was expecting more for my money. I told the machinist to deck it .020" which should leave me .005" deck clearance. What I got was not even close and all cylinders vary quite a bit! See the attached image for my measurements. My goal is to have a nice tight squish and these numbers are screwing with my plans.

Pistons are H345ACP60 which have the standard compression height (1.560").
The rods are factory, used and reconditioned.
The crank is factory, used, and reground.

I understand that there is rod stretch, and after reconditioning the rods and regrinding the crank there may be some variances in rotating height, but I think what I am seeing here is crazy. I am not a classically trained machinist. I used a bolt down dial indicator to bring the pistons to exact TDC. Then used a straight edge on the block and slipped some feeler gauges between the edge and piston (front and rear of the piston, parallel to the wrist pin to factor out piston rock). Yeah there maybe a +/-.002" at the most in my measurements, but I doubt it.

1) Are these pistons known to be a little funky in the exact compression height as well as a nice level wrist pin hole?
2) Are these numbers normal for a "regular" machine shop (I am not building a Swiss watch or F1 engine)?
3) Is the difference between and left and right side due to core shift? Should I use a thinner gasket on the left side to compensate?
4) How could he have taken off .020" and the pistons be so far down the hole (advertised piston compression height not accurate)?
6) Should I just use the average numbers to figure out my compression ratio and squish?
7) For you guys that zero deck your block, do all your pistons come to exactly .000" clearance?
8) Any advice on what to do?
Quote:
The rods are factory, used and reconditioned.
GM rods after being resized are all over the place depending what the shop did to them and its always manditory to have the rods checked for bend or twist after being resized.

Also the crank probably was not indexed ground.

Using OEM parts they are all over the place!!!!

Quote:
I told the machinist to deck it .020" which should leave me .005" deck clearance
You should have tool your machinist you wanted 8.995 deck height if he took .020 off the decks like you asked that is what he did OEM decks are all over the place.

The shop doing the work should be using a BHJ squaring plate to the job right and should have good equipment and measuring tools to do a good job.

At .060 over hopefully you had your block sonic tested as I don't find many GM block safe at .060 for a performance build!! At That bore size a plate hone is must as well!!!

Zero deck is zero oil clearance that being said most companies that make rods and pistons have a plus or minus tolerance of .001 and also depending on the quality of the crank being used it those could be off as well.

Using quality parts I see a .0015 difference in deck heights. On SBC 421 build I did a few weeks ago was with in .0005 every where.

At our shop we use quality parts and I use a HAAS 4-axis CNC machining center to do our block machining we don't have to waste time mocking up rotators as you can't make much money doing it that way!!!

If a shop is mocking up a block and decking to the demensions found from front to rear on pistons heights there is a good chance the block is not being squared of the center line of the crank which is a no no.

Last edited by CNC BLOCKS NE; 04-09-2012 at 06:08 AM.
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Old 04-09-2012, 01:56 PM
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Quote:
Originally Posted by Silver Surfer
This is my first engine build up (SBC 350). Perhaps I was expecting more for my money. I told the machinist to deck it .020" which should leave me .005" deck clearance. What I got was not even close and all cylinders vary quite a bit! See the attached image for my measurements. My goal is to have a nice tight squish and these numbers are screwing with my plans.

Pistons are H345ACP60 which have the standard compression height (1.560").
The rods are factory, used and reconditioned.
The crank is factory, used, and reground.

I understand that there is rod stretch, and after reconditioning the rods and regrinding the crank there may be some variances in rotating height, but I think what I am seeing here is crazy. I am not a classically trained machinist. I used a bolt down dial indicator to bring the pistons to exact TDC. Then used a straight edge on the block and slipped some feeler gauges between the edge and piston (front and rear of the piston, parallel to the wrist pin to factor out piston rock). Yeah there maybe a +/-.002" at the most in my measurements, but I doubt it.

1) Are these pistons known to be a little funky in the exact compression height as well as a nice level wrist pin hole?
2) Are these numbers normal for a "regular" machine shop (I am not building a Swiss watch or F1 engine)?
3) Is the difference between and left and right side due to core shift? Should I use a thinner gasket on the left side to compensate?
4) How could he have taken off .020" and the pistons be so far down the hole (advertised piston compression height not accurate)?
6) Should I just use the average numbers to figure out my compression ratio and squish?
7) For you guys that zero deck your block, do all your pistons come to exactly .000" clearance?
8) Any advice on what to do?
My guess would be that you've got 1.54 compression height rebuilder pistons in spite of what your part number says they are, in a zero deck block which mostly restores the original clearance that was there with a 1.56 inch production piston in a standard deck block. These days you can't accept what the part number says, you've got to measure there is so much crap out there it's enough to drive you to drink. The right to left variance that you do see is probably the crankshaft bore not on center with the cylinder bores, a common event.

Bogie
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Old 04-09-2012, 02:03 PM
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Quote:
Originally Posted by oldbogie
My guess would be that you've got 1.54 compression height rebuilder pistons in spite of what your part number says they are, in a zero deck block which mostly restores the original clearance that was there with a 1.56 inch production piston in a standard deck block. These days you can't accept what the part number says, you've got to measure there is so much crap out there it's enough to drive you to drink. The right to left variance that you do see is probably the crankshaft bore not on center with the cylinder bores, a common event.

Bogie
The pistons are advertised as std comp height. I will go back and double check my measurements, hopefully I will have time tonight.
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Old 04-09-2012, 02:33 PM
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All of the Sealed Power H-series pistons used to be .020 short. Dont know about the 345ACP
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Old 04-09-2012, 02:37 PM
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Here is what I got. Says 1.560", maybe its a typo?
http://www.summitracing.com/parts/SLP-H345ACP60/
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Old 04-09-2012, 03:55 PM
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Sealed Power catalog says 1.548".
http://www.fme-cat.com/PartDetailWin...Number=H345ACP
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Old 04-09-2012, 04:14 PM
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Quote:
Originally Posted by lmsport
Thanks! I have been trying to confirm this number. I called Summit and they looked it up in their master catalog and confirmed it was 1.560".

I think I found the corresponding pistons with factory CH: they are H669ACP

Both H669ACP and H345ACP are listed with Summit at 1.560"

Arg *********************** Summit!
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Old 04-09-2012, 04:27 PM
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Quote:
Originally Posted by lmsport
Yes I know what the catalog says, the question is did you actually get the physical part the catalog and probabaly the packaging says you got. In other words were the pistons measured before being installed.

The other option is the deck didn't get zeroed and the pistons are what they are claimed to be.

It isn't likely that rod and crank dimensional errors would make numbers so close to the OEM clearance so consistently. Production variation is tracked statistically and that would fly in the face of this much consistency if that were the case. The random nature of statistical variation is against this occurrence unless all your rods came as a group through all the manufacturing processes from forging to final machining. Possible but improbable.

Therefore; this loops back around to the pistons either not being the part number they claim to be, or were mis-machined as a batch, or the decks really didn't get cut.

Bogie
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Old 04-09-2012, 05:56 PM
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Recon rods vary alot.
Reground cranks, especially from someplace like Standard can vary alot in stroke and index. I had a 455 Olds crank that was off .020 on one journal and that was considered OK.
There is no telling where the deck was in relation to the main line, or where it is now.
I bet the pistons dont vary .001 in deck height, .0002 in diameter and no more than 3grams in weight.
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Old 04-09-2012, 08:07 PM
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Your crank was reground on its original centers, hence your mismatch. Mix and match rod/piston assy's to alternate bores to equalize the measurements...thats what everyone else does.

Usually one bore ends up short, make that bore the typical lean cylinder for your engine.

Don't get too hung up on the small irregularities...in the end it won't matter even 1 horsepower at the power levels you seek.
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Old 04-09-2012, 09:05 PM
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OK I re-checked the numbers using a different technique and they all came back very similar if not the same. In fact when I average the first set of numbers, the second set larger by .001" on both left and right cylinder banks.

Left bank average is .022-.023"
Right bank average is .019-.020"

If the machinist took .020 off and the pistons are now known to be 1.548" CH, then that leaves a calculated clearance of .017". Oddly enough, the only cylinder to do this is the one that has the replacement rod. It is constant on the fore and aft measurement.




Quote:
Originally Posted by 4 Jaw Chuck
Don't get too hung up on the small irregularities...in the end it won't matter even 1 horsepower at the power levels you seek.
I know this probably seems trivial to you. But I am newbie and the only way I am going to learn how to build a proper engine is to sweat the details. If I don't pay attention now, then what good is it for my next engine? Perhaps part of the learning curve is selecting a good machinist and dealing with the ensuing disaster.
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