Piston in the hole question. - Team Camaro Tech
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post #1 of 9 (permalink) Old Aug 20th, 03, 06:30 AM Thread Starter
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I have an assembled 350 .030 over short block with flat tops and 4 valve relief’s. I used a dial indicator to get the piston TDC then placed a steel straight edge across the cylinder and used a feeler gauge to check the distance from the top of the piston to the straight edge. First off is this an acceptable way to measure and secondly is .040 in the hole normal. From what I've read .040 seems like too much. I checked two pistons, one on each side.

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post #2 of 9 (permalink) Old Aug 20th, 03, 07:09 AM
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rojo;

.040 does seem a bit much, I usually see @.030~.025 on 'stock' flat tops.
I have used a straight edge and feeler to get good readings with no problems as long as the block deck is clean and flat. If you have a dial indicator you can use it to check the deck height by taking a reference reading on the deck and 'zero' the indicator - then swivel the base 90deg. and read the piston TDC. The difference would be the clearance.
The good news is you can use a thin shim head gasket to help get the clearance down, increase the compression ratio and increase the 'quench'if you want. FelPro makes a good coated shim (#1094).
Hope this helps.

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post #3 of 9 (permalink) Old Aug 20th, 03, 08:17 AM
 
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Ditto on the stock around .025" in the hole... pdq67



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post #4 of 9 (permalink) Old Aug 20th, 03, 01:08 PM
 
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If you are using a replacement piston you will be around .040 down in the hole. I had a set of 345np cast pistons and they were .045 in the hole. I checked the stroke and it was dead on. I called Federal Mogul and talked to a tech guy and he said that they make the replacement pistons with a reduced compression height to compensate for block and head milling. If you go to use a .040 gasket that really makes for a poor quench.

Before the milling the motor had 8.1:1 compression and ran pretty soft all around. After I had the block milled .045 it had 9.2:1 and ran much harder and got better mileage as well. It made a world of difference. I don't know if it was the added compression, or if it was better quench.
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post #5 of 9 (permalink) Old Aug 20th, 03, 10:16 PM
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also in addition to what has been said, when you take the measurement make sure its in the center of the piston, so the rocking of the piston doesn't skew the results. they shouldn't rock much, but it can effect the measurement. you may already know this, but just thought i'd throw it out for consideration [img]graemlins/thumbsup.gif[/img]

Sean

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post #6 of 9 (permalink) Old Aug 21st, 03, 03:15 AM Thread Starter
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Thanks for the replies.

I didn't think about just swinging the base of the dial indicator around, I'll give that a try but I bet it's going to be close to the same results. I did measure in the center of the piston but just by coincidence, I didn't think about piston rocking. As far as replacement pistons that could be, this engine was in the car when I bought it so I don't know the history. The short block seems in good shape and I was considering putting it back in with some decent heads and cam. Even with a .015 head gasket I'll be at .055 quench which I guess is still a little high.

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post #7 of 9 (permalink) Old Aug 21st, 03, 09:01 AM
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Well actually if you fiqure a composion gasket at @.040" (plus) compressed height, then you are just about right where you would be if you have .030~.025" deck clearance and a comp. gasket just by using a shim type.
'6 of one & 1/2 dozen of the other'
You are giving up maybe .010~.015" on the deck and gaining .025" on the shim gasket. I could see where you are going to the plus side of things in "stack Height" by up to .010"
This not an uncommon problem, and it is something I always check when someone brings me an engine "that just doesn't run like it should".
Good luck - post back with your results.

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post #8 of 9 (permalink) Old Aug 23rd, 03, 08:11 PM
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Sounds like you got screwed. Many cheaper flat-top pistons are made too short so high-volume rebuilders can slap motors together as fast as possible without wasting time worrying about things hitting each other. Federal-Mogul or Speed Pro or whatever their name is this week has several of these, as do most other manufacturers, and some people sell them without telling customers that the compression height is up to .020" less than what it should be.
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post #9 of 9 (permalink) Old Aug 23rd, 03, 08:54 PM
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Most cast rebuilder pistons are made .020 shorter on purpose...to compensate for head/block milling. It does screw up the quench though if you don't deck the block or just need minimal material removed to get a flat surface. The engine in my truck runs TRW/federal mogul cast flat tops, and with .008 cut off the deck they ended up .040 in the hole. Thats why I only have 9.4-1 compression with a flat top piston and 64cc head. You can run a fel-pro 1094 head gasket (.015 thick) to help in the quench/compression department.

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