# Headgasket on or off to check pushrod length?



## Shake-N-Bake (Jun 16, 2015)

Tomorrow I will bolt on the head and use a Comp Cams adjustable pushrod to determine the length I need. 

The FelPro technical information says the compressed thickness of the head gasket I am using is .004-.005". My plan was to bolt on the head without the gasket, determine what length I need and then add the 4 or 5 thousandths to that measurement.

Any problems with this plan? I would prefer to torque down the head gasket just once if possible.

















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## bigD (Jul 21, 2016)

I think maybe you have the decimal point in the wrong place. Fel-Pro 1016 gaskets are said to be .039 thick & the 8518PT gaskets are said by some to be aprox .045 - .048 thick.

https://www.summitracing.com/parts/fel-1016

https://www.summitracing.com/parts/fel-8518pt

This may be where the .004 & .005 numbers came from.

http://www.felpro-only.com/break_room_forum/showthread.php?tid=1082


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## PontiacJim (Dec 29, 2012)

Why not wait until you bolt the heads on and eliminate any guess work? 

Felpro offers 2 head gaskets. Here is what info I have on them from Rocky Rotella's Pontiac book:

Fel-Pro introduced PN-8518 for the Pontiac V-8 during the 1970s. Containing a bore diameter of 4.3 inches and a preflattened thickness of .041 inch, it’s a stock-replacement design that’s compatible with most Pontiac blocks. an affordable choice for any stock rebuild, and even performance applications with a compression ratio up to 10.5:1, or slightly more.

A second Pontiac offering appeared in the Fel-Pro catalog during the early 1980s. PN-1016 is specially designed for high-performance applications with high compression and/or combustion pressure. Constructed of a Teflon-coated solid-steel core, it features stainless steel combustion armor and a wire ring combustion seal for maximum combustion containment. It contains a chamfered bore diameter or 4.3 inches and pre-compressed thickness of .039 inch. it’s a good choice for naturally aspirated engines with a maximum compression ratio of 12:1 and/ or mildly boosted applications.

These gaskets, he says, have larger than factory water passages and are close to the sealing ring which could be a problem *IF* the seal around the cylinder were to let go.

The book states the Butler offers a composition gasket, .045" when compressed, that has the factory sized water passages. Butler recommends a light coating of copper sealer on the steel side for maximum block seal. Good for engine up to 11.5:1 and mild boosted engines. https://butlerperformance.com/c-1234803-gaskets-head-gaskets.html


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## Shake-N-Bake (Jun 16, 2015)

bigD said:


> I think maybe you have the decimal point the wrong place. Fel-Pro 1016 gaskets are said to be .039 thick & the 8518PT gaskets are said by some to be aprox .045 - .048 thick.
> 
> https://www.summitracing.com/parts/fel-1016
> 
> ...


You are correct. I misread the tech sheet. (Was getting late when I posted...)
Sheet says the gasket is .042"-.048" and compresses down another .004"-.005" when torqued. So I guess I need to measure the uncompressed thickness and do the math.


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## Shake-N-Bake (Jun 16, 2015)

PontiacJim said:


> Why not wait until you bolt the heads on and eliminate any guess work?
> 
> Felpro offers 2 head gaskets. Here is what info I have on them from Rocky Rotella's Pontiac book:
> 
> ...


Thanks Jim.
I have light weight checking springs installed so I have to remove the heads after I determine the push rod length. The Comp Cams adjustable push rod shouldn't be used with full spring pressure, according to the instructions.

The gasket I have is 8518 PT.

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## bigD (Jul 21, 2016)

Those numbers listed from the RR book don't jive at all with other published specs for those 2 FP head gaskets. 

I have no idea what the actual specs are. Anybody here ever actually measure some of both ?


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## Shake-N-Bake (Jun 16, 2015)

The 8518 gaskets in my kit measured .047". Installing the head with bolts snug (to check piston to valve clearance) compressed it .0005".

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## PontiacJim (Dec 29, 2012)

bigD said:


> Those numbers listed from the RR book don't jive at all with other published specs for those 2 FP head gaskets.
> 
> I have no idea what the actual specs are. Anybody here ever actually measure some of both ?


Agreed, seems each builder or seller has a different spec, but Shake-N-Bake measured his so we know the 8518 measures .047" uncompressed. BUT, did he use a manual micrometer or digital? Was it made in the US or China? And what exactly was the ambient air temperature and humidity when he did his measuring at what altitude? :yesnod:


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## Shake-N-Bake (Jun 16, 2015)

I measured with a Starrett micrometer. About 70 degrees ambient....30% humidity.  the metal ring around the bores were thicker....about .051"
Makes sense actually.

So, in the end, I had to do what Jim said. I just couldn't get the pattern on the valve stem tip the way I like it. The Comp Cam adjustable push rod was difficult to work with. It wouldn't hold its length setting and was giving me some weird patterns. I had to tape it up for each try so the length would remain constant. That fix helped but I still had some troubles. Eventually I figured out the Comp Cam push rod was 5/16" dia but my guide plates were for 11/32" rods (since my engine is a RA II)

Lastly, the light weight checking springs really were not putting enough pressure on the valve stem tip to rub away the marking ink. So, I decided to put the regular valve springs back on the heads and bolt the heads to the block...with the head gasket in place, like Jim suggested. The Comp cam pushrod worked fine with the higher spring pressure.

So, in the end it turned out that I needed 9.170" push rods.....same length as my stock RA II push rods so I don't need to order a custom length. I pretty much wasted a half day with this whole exercise but at least now I know the stock push rods are going to work fine.










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## RMTZ67 (Mar 12, 2011)

*comp cam guide plates*



Shake-N-Bake said:


> I measured with a Starrett micrometer. About 70 degrees ambient....30% humidity.  the metal ring around the bores were thicker....about .051"
> Makes sense actually.
> 
> So, in the end, I had to do what Jim said. I just couldn't get the pattern on the valve stem tip the way I like it. The Comp Cam adjustable push rod was difficult to work with. It wouldn't hold its length setting and was giving me some weird patterns. I had to tape it up for each try so the length would remain constant. That fix helped but I still had some troubles. Eventually I figured out the Como Cam push rod was 5/16" dia but my guide plates were for 11/32" rods.
> ...


I had bought a set of comp cam guide plates and measured them against my stock ones. They were larger, Just can't remember the figures so I am gonna use my stockers.


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