Minimum piston/head clearance (quench)

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sarnett346
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Minimum piston/head clearance (quench)

Post #1 by sarnett346 » Thu Jun 14, 2018 5:46 am

Hello all,
What would be the absolute minimum piston to head clearance on a 200 cubic inch 6?
I'm using flat top pistons with a zero deck height, and an aluminum head. I'd like to get as thin of a head gasket as possible. Would .032 to 0.40 be doable without catastrophic results?
Thanks for any info!
-Sean

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bubba22349
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Re: Minimum piston/head clearance (quench)

Post #2 by bubba22349 » Thu Jun 14, 2018 8:18 am

Commonly available head gaskets are in a .044 or .050 thickness, to go with less would require a custom made head gasket when your staying with a zero decked block. A .035 to maybe .030 is about the practical minimum Qunech. Besides a custom head gasket one other way is going with a positive piston deck height. Ie Custom Pistons, more block deck machining, and or with an offset grind on the crank (also giving you a little more Cu. In. too), this would have the Pistons sticking up past the blocks deck .009 to .014. Good luck on your 200 build up :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

sarnett346
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Re: Minimum piston/head clearance (quench)

Post #3 by sarnett346 » Thu Jun 14, 2018 11:01 pm

Thanks for the info! So, a .030 clearance should be safe to run up to say... 6000 rpm? How tight to my bearing clearances need to be?

On a different, but related note; who makes a good custom head gasket? What am I going to be looking at price wise?

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bubba22349
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Re: Minimum piston/head clearance (quench)

Post #4 by bubba22349 » Fri Jun 15, 2018 8:26 am

Bearing clearances are determined by the engines intended use. Ie Street driver, Full Race track car, or a combo of those two. Cometic might make you a custom head gasket I don’t have any idea as to the cost. You also used to be able to get Copper head gaskets made. One last thing if you happen to find a rare Ford NOS steel shim head gasket for a 200 they are .025 thick, so then your quench could be down to .025 too. Good luck :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Minimum piston/head clearance (quench)

Post #5 by Econoline » Fri Jun 15, 2018 10:45 am

This later composite ford head gasket is easier to find than the steel ones. These are .035" iirc. They are often found as a Ford NOS "valve grind kit".
https://www.ebay.com/itm/NEW-OUT-OF-BOX ... Sw-W9bIEz5
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Re: Minimum piston/head clearance (quench)

Post #6 by CNC-Dude » Fri Jun 15, 2018 11:26 am

sarnett346 wrote: I'd like to get as thin of a head gasket as possible. Would .032 to 0.40 be doable without catastrophic results?
Thanks for any info!
-Sean


Many people don't have a true understanding of how to correctly check the pistons true deck height to be safe, or where to correctly check them. A true zero deck height with a .040" head gasket can give you an actual piston to head clearance of only .010" at the quench and spark plug locations, which is the most critical areas to check. I have seen piston to head contact with a .030" head gasket and zero deck. If you are measuring your pistons deck height at the center of the cylinder bore, this is completely dangerous and can/will cause severe engine failure because this isn't where the piston is the closest to the cylinder heads flat deck area. When people ask or say "what is the optimum or minimum quench", they don't realize that it is this area of the cylinder where you actually check the deck clearances on the pistons, that's why it is called "quench", and that is where you measure the deck height, not the center of the cylinder or piston.

If you think about it, what is directly above the piston in the center of the cylinder bore relative to the cylinder head.... the vast openness of the combustion chamber. So the deck height of the piston in this area really has no relevance or meaning to establishing the quench distance would it? Finding and measuring the height in the center of the piston is only the starting point of the process, and many people also don't realize this is only the starting point of the process. Once you get the piston to TDC, measured with a dial indicator, you then move the indicator to the far outside edge of the pistons on both the quench side and then over to the spark plug side of the pistons. Then you rock the piston as hard as you can from side to side and then take the average of the two readings at each spot. This simulates what the piston is doing when the engine is running. The pistons will rock out of the cylinders an average of .010"-.015" giving you a positive or above the deck reading.

The actual amount the pistons will measure can/will vary from cylinder to cylinder because of many variables such as piston to cylinder wall clearance(race and performance engines will be more because they have more cylinder wall clearance), varying connecting rod lengths due to manufacturing tolerances and crank stroke on each journal, also due to manufacturing tolerances, and also the deck squareness of the engine block both side to side and end to end. And then the wrist pin location also considering the machining tolerances. Some of these variances are also made worst by previous bad machine work from earlier rebuilds. Then once you factor in the thermal expansion process when the engine is at operating temps, that zero deck clearance becomes even less. Point being, you can not just check one piston and rod in one cylinder and then deck the block to assume you will be ok. Trying to achieve zero deck without actually physically checking each rod and piston in their respective cylinder bores puts you dangerously close to the edge of a cliff, and you are often unaware of how close to engine damage you actually are. You also have to flycut most of the pistons to correct the cylinder to cylinder variance you have to equalize the readings as well.
Last edited by CNC-Dude on Sat Jun 16, 2018 4:01 pm, edited 1 time in total.
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Re: Minimum piston/head clearance (quench)

Post #7 by CNC-Dude » Fri Jun 15, 2018 1:04 pm

....
Last edited by CNC-Dude on Sat Jun 16, 2018 3:56 pm, edited 1 time in total.
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Re: Minimum piston/head clearance (quench)

Post #8 by drag-200stang » Sat Jun 16, 2018 10:57 am

Very nice on the money detailed post, CNC.. :thumbup:
66 Mustang Coupe
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Re: Minimum piston/head clearance (quench)

Post #9 by CNC-Dude » Sat Jun 16, 2018 11:04 am

Thanks, I like to try and give people a good foundation so they can achieve good results in their projects. Hope it's been helpful to others as well.
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