Where Can One Find Published Factory 360 Piston Compression Height?

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dibbons

dibbons

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My internet search came up with nothing in writing anywhere, don't know why the big secret. Where is the darn compression height published for the factory 360's anyway? Thank you.


Foto is not 360, for illustration purposes only:

compression height.png
 
"should" be in factory service manual. and I would "think" that should be the same except for '71 with flat top pistons.
 
I found this chart on-line in a "for sale" book titled "How to Rebuild Your Small Block Mopar" (by Don Taylor and Larry Hofer) which lists "H" compression height for 360's as 1.584" and 1.591". In post #3 above, that web page listing shows the 360 compression height as 1.687". So, who is right? Maybe no-one. Who has a factory 360 piston and a good eye for measuring with precise instruments?

Another problem, the list I found shows "D" block deck height for all small blocks as 9.94" and I believe that figure is more like 9.60"/9.599". It is hard to believe anything you read. I still find no factory authoritative source and the factory service manual's show nothing with respect to piston compression height.

How to rebuild 2.06.52 PM.png
 
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BigBlockMopar said:
Not everything is a secret if you just can't find something... ;)
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Yes, I found that page too, but it is bogus as far as I am concerned until someone can actually measure a 360 factory piston accurately or find the spec list from Chrysler Corporation. I came on the forum to find help to clarify the specifications accurately, not to have some smart *** criticize the beginning of my investigation to find the truth.
 
Pic is from my 84 Dodge truck fsm. It gives overall 360 piston length as 3.19" if that helps any. 318 is 3.47"

20171205_204906.jpg


20171205_204900.jpg
 
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dibbons said:
In post #3 above, that web page listing shows the 360 compression height as 1.687". So, who is right? Maybe no-one.
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That web page listing (Victory Tech Papers) shows 'maximum compression height' which is for a theoretical zero deck piston. That is not actual compression height of a stock piston.

dibbons said:
Another problem, the list I found shows "D" block deck height for all small blocks as 9.94" and I believe that figure is more like 9.60"/9.599".
Click to expand...
The latter number is correct. You can use that with confidence.

The table you showed has some numbers right but others wrong so it is correct to question that table.
 
1.59 inches; source: "How to Hot Rod Small Block Mopar Engines" by Larry Shepard
 
The problem is, as you eluded to, that there is more than one measurement. I think there is even more than that. Add in the Magnum engines and we have even more measurements to throw in, since the Magnums have a shorter deck height. The Larry Shepard info JBurch posted is probably the best guess so far.

Through the years, many car companies changed compression heights to deal with ever changing fuel requirements. Ford, for example had over FIVE different compression heights for their 400 engine from its inception in 1970, to its demise in 1982, trying to combat detonation.......and they never succeeded, because they were going the wrong way! Had they used a closed, quench chamber and zero deck height, they would have beaten the problem, but little was known about quench benefits back then. Same I am sure with all the other car manufacturers, as well.

IMO, this is one reason you find so many "correct" compression distance figures.

My answer is simple......and not trying to be a smartass, but pick one. It does not make "that"much difference. I would go with the longest compression distance that's "around" whatever the nominal spec is. This will keep block decking to a minimum to achieve zero deck height, IF that's what you are going to do.
 
Just out of curiosity, why do you want to know the exact compression height of an old factory stock piston? I wouldn't worry about it if you are building an engine, take your own measurements, never assume, and then select the proper piston for your application and desired compression ratio. I apologize if you already know this, perhaps you are just curious as to why you cant find the info easily on line. Personally, I like to look up specs from old books, more so because my hands are usually dirty and searching the web when I have tools in my hands may result in an intentionally broken laptop.
 
RustyRatRod said:
The problem is, as you eluded to, that there is more than one measurement. I think there is even more than that. Add in the Magnum engines and we have even more measurements to throw in, since the Magnums have a shorter deck height. The Larry Shepard info JBurch posted is probably the best guess so far.
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There are also truck compression pistons (lower compression and CH numbers) to add to the confusion, in addition to aftermarket pistons often being a bit different from stock factory numbers. Look up 318 LA pistons in the Silvolite catalog.... you will find 2-3 different CH's.

One thing to NOT do: Don't ever take data from a distributor's web page..... the odds of being incorrect are the highest there.
 
I finally found several of my OE 360 pistons & with a steel scale I got 1.648" & with my HF green calipers I got 1.601". With that much disparity, I will get a more accurate # in a day or two when I get back out there.
 
@krazykuda

What say you Karl? You worked for Chrysler. You got any info on this?
 
Rapid Robert said:
I finally found several of my OE 360 pistons & with a steel scale I got 1.648" & with my HF green calipers I got 1.601". With that much disparity, I will get a more accurate # in a day or two when I get back out there.
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Good to see that...but that disparity is indeed a bit of a problem!
 
dibbons said:
I found this chart on-line in a "for sale" book titled "How to Rebuild Your Small Block Mopar" (by Don Taylor and Larry Hofer) which lists "H" compression height for 360's as 1.584" and 1.591". In post #3 above, that web page listing shows the 360 compression height as 1.687". So, who is right? Maybe no-one. Who has a factory 360 piston and a good eye for measuring with precise instruments?

Another problem, the list I found shows "D" block deck height for all small blocks as 9.94" and I believe that figure is more like 9.60"/9.599". It is hard to believe anything you read. I still find no factory authoritative source and the factory service manual's show nothing with respect to piston compression height.

View attachment 1715118238
Click to expand...


RustyRatRod said:
@krazykuda

What say you Karl? You worked for Chrysler. You got any info on this?
Click to expand...

Yes Rob, here is what I have found:


That info is not accurate if you do the calculations... Those are book numbers, and not actual...

I tried to do the compression calculations with those numbers and they don't work out... The only way to know for sure is to measure an actual set...

I have a set of 67 920 heads which are listed to be 57 cc in that book (I have a copy of it myself) but actually measure 63 cc's after my head guy went through them and may have done a clean up mill cut to square them back up... You can't trust the book numbers - they do not add up...

The X and J heads are listed at 65 cc and I've measured them at 70 - 72 cc...

To calculate your true compression for your particular set of components, see my How-to Check Compression thread:

How to Check Compression


Chrysler likes to run to the fat side of their tolerances...

Example: The engine that I worked on had aluminum heads and cast iron block... If there was a complete engine with a defect that couldn't be shipped, it went to be torn down and the parts recycled... The heads would sometimes get scratches on the head gasket face and need to be re-milled to get the scratches off and the surface back to the smoothness spec... So we ran the first time heads just to the top (fat side) of the tolerance so we could do a mill cut or two to bring them back to usable condition... We would save them until we had a few pallets of them and batch them through the head line where the block face was re-milled... We would adjust the machine to run a light clean up cut on that face, then set it back up for the first time through heads afterwards... It saves us from having to scrap heads for light scratches in them...

We also ran the block head decks to the top of the tolerance for the same reason, but they weren't scratched as easily as the aluminum surfaces on the heads as they were cast iron...

That early edition of How to Rebuild a Small Block Mopar is a great book, I've had one and used it for years and learned alot from it especially when I first started out building engines... I like the early edition with the gold cover better than the revised edition as they removed the parts ID chapter in the later editions... A must have in my opinion for anybody starting out on SB LA engines...
 
I believe those smaller head chamber volumes are NHRA minimum volumes ....which may have come from Mopar at some point, and which may have been used for the 'higher than reality' factory compression ratio numbers.
 
Good to see that...but that disparity is indeed a bit of a problem!
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Agreed! so I braved the numbing wind chills/blowing snow/howling wolves (& turned off the TV) & went out there again & measureing twice I got 1.1093" and 1.1171" from pin to top then I added in half of the pin (.492") for final readings of 1.601" and 1.609"
 
Taking the average of your 2 numbers at 1.605" and adding half the stroke plus the 6.123" rod length gives 9.518" which is about .082" below standard deck height.... which matches up pretty well with reports of measurements of piston tops below deck at TDC. (I've been using .070"-.080" below deck for 360 computations with stock pistons.)
 
nm9stheham said:
Taking the average of your 2 numbers at 1.605" and adding half the stroke plus the 6.123" rod length gives 9.518" which is about .082" below standard deck height.... which matches up pretty well with reports of measurements of piston tops below deck at TDC. (I've been using .070"-.080" below deck for 360 computations with stock pistons.)
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It sure does!

And all this work and all these responses for what appears to be a drive by post. Oh well. At least the rest of us can learn somethin!
 
Thanks Karl! I knew you'd have some good input.

krazykuda said:
Yes Rob, here is what I have found:


That info is not accurate if you do the calculations... Those are book numbers, and not actual...

I tried to do the compression calculations with those numbers and they don't work out... The only way to know for sure is to measure an actual set...

I have a set of 67 920 heads which are listed to be 57 cc in that book (I have a copy of it myself) but actually measure 63 cc's after my head guy went through them and may have done a clean up mill cut to square them back up... You can't trust the book numbers - they do not add up...

The X and J heads are listed at 65 cc and I've measured them at 70 - 72 cc...

To calculate your true compression for your particular set of components, see my How-to Check Compression thread:

How to Check Compression


Chrysler likes to run to the fat side of their tolerances...

Example: The engine that I worked on had aluminum heads and cast iron block... If there was a complete engine with a defect that couldn't be shipped, it went to be torn down and the parts recycled... The heads would sometimes get scratches on the head gasket face and need to be re-milled to get the scratches off and the surface back to the smoothness spec... So we ran the first time heads just to the top (fat side) of the tolerance so we could do a mill cut or two to bring them back to usable condition... We would save them until we had a few pallets of them and batch them through the head line where the block face was re-milled... We would adjust the machine to run a light clean up cut on that face, then set it back up for the first time through heads afterwards... It saves us from having to scrap heads for light scratches in them...

We also ran the block head decks to the top of the tolerance for the same reason, but they weren't scratched as easily as the aluminum surfaces on the heads as they were cast iron...

That early edition of How to Rebuild a Small Block Mopar is a great book, I've had one and used it for years and learned alot from it especially when I first started out building engines... I like the early edition with the gold cover better than the revised edition as they removed the parts ID chapter in the later editions... A must have in my opinion for anybody starting out on SB LA engines...
Click to expand...
 
Now I gotta ask Rapid Robert what the dish pattern was on top of his 360 pistons. I ask because the Silvolite catalog lists his CH of 1.605" as being for Magnum vintage engines ('93-'01). Those have a sorta-rectangular shallow dish, like 2 D's back-to-back. For the earlier LA, Silvolite lists the CH as 1.576 and show either flat tops or circular dish tops. FME also lists 1.576" as the CH for the LA 360 pistons. (Sealed Power 405 pistons, with the circular dish.)
 
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