Deck Clearance Clarification

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Jun 29, 2004 | 12:06 PM

Ryan356

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From: Watertown, NY

Deck Clearance Clarification

I have been doing a lot of research on my engine rebuild....over a year now and im almost ready to get the parts. I am going to be taking my 88 351 block and putting a 393 stroker kit in it with gt40p heads on it for now that are parted and polished. The thing I have always been fuzzy on was what deck clearance actually was and what changed it. I originally thought that it depended on the block and year but someone at work told me that most blocks were the same and that the piston/crank setup is what determines the deck clearance. So am I right in thinking that two 351s with different deck clearances has the difference because the pistons themselves are different thicknesses? Im just not sure how this works. I was hoping someone could clarify. I do understand that deck clearance is the space at TDC between the top of the piston and the actual top of the cylinder. So maybe this is a dumb question. How is it effected....by the crank, rods, piston thickness, or all...and are all blocks generally the same? I was thinking the later was the case because piston companies can advertise the expected CR based on the block. Thanks for any help and I hope you can understand this.

Jun 30, 2004 | 07:40 PM

ga302p

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What determines deck clearance. Actual deck height of block, and rod length are the variable you will see change it most often. If a stroker crank was built to increase compression drastically, it would slightly change it. Piston affects deck clearance. Some pistons have the wrist pins moved down to increase compression ratio, some have the moved up to lower it.

The gt40-p head is extremely over rated. No way i would use it on a stroker unless it was "radically" ported. Not just a little port work, a lot.

Jul 1, 2004 | 07:58 AM

Ryan356

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From: Watertown, NY

So I was right in thinking that all those factors make a difference in the deck clearance... So if im building up my '88 351w do I need to find the deck height for that specific year or were they all the same on that block? Something tells me they are all gonna be the same and that the rotating assembly changes are what made the deck clearance numbers differ. The reason why I ask is because I want to run on pump gas and the stroker kit/w dished pistons says that I will be at 9.5 to 1 CR with a combustion chamber volume of 60cc's and it never mentions anything about deck clearance changing anything. The gt40's are right around there, so I think I might be ok. Also, the gt40p's are only temporary until I get some afrs or twisted wedge heads. Thanks for the help.

Jul 1, 2004 | 09:12 AM

Mike G

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From: Littleton, CO

The deck clearance "should" be the same on all motors, but then again each one is separately cast. Your assumptions are just that with the advertised information. The only truth will be to measure your individual deck height.... You can also change your formula by using different head gaskets. Most steel shim type are .020 and the heavier soft ones are .040. Do it and have fun, you can always adjust for too much compression with minimal changes.

--Mike

Jul 1, 2004 | 11:16 AM

Ryan356

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From: Watertown, NY

Thanks for the help guys. After some searching of the internet about this I found out that the deck height is the same for my block way back to 1971 when there was a small change. So my conclusion is that deck height changes from year to year are because of rotating assembly changes not the block like I was wondering. This is why companies can advertise the compression ratios expected from that block. I still dont know how to find the actual deck clearance on my motor. Im thinking I have to take the total deck height and subtract the height of the piston and the rod at centerline of crank... In any case it doesnt really matter I was just curious and it would be nice to understand things more in depth as I attempt my first buildup.

Jul 1, 2004 | 11:51 AM

Mike G

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From: Littleton, CO

Just so long as the valves don't hit the pistons, it'll run

You will need to mock up the engine and measure for exact deck clearance, other than that it's just formulas and no definites. Of course milling will also need to be taken into account if your deck has been resurfaced.

Enjoy --Mike

Last edited by Mike G; Jul 1, 2004 at 11:54 AM.

Jul 1, 2004 | 06:29 PM

ga302p

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IF you are considering updating to a twisted wedge head, be sure you use a piston that is made for them. I have never built an engine for the twisted wedge heads. Mainly because they are not as good as some of the other performance heads on the flow bench, and the are pricey. The valve reliefs have to be twisted in the piston also. In your stroker, the dish may be able to handle the valves without any reliefs, just something you would have to check.