Can i run low head with domed pistons

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JohnnyBuick
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Can i run low head with domed pistons

Post by JohnnyBuick » Mon Nov 16, 2020 10:33 am

I pulled off the high head on my 1912 engine today and i noticed the engine is fitted with fully domed piston heads . The domed pistons run approx 1/2 inch above the block deck. If i was to change to a low head could it become a mechanical catastrophe ?
...An idle mind is the Devils workshop :twisted:

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TWrenn
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Re: Can i run low head with domed pistons

Post by TWrenn » Mon Nov 16, 2020 2:56 pm

I'm not a mechanic but I'll get this back to the top for more experienced guys to discuss, but I would say yes it would be disastrous. Seems to me there's not enough clearance. Maybe I'm wrong, but it's a gut feeling!


Kerry
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Re: Can i run low head with domed pistons

Post by Kerry » Mon Nov 16, 2020 3:15 pm

I've never had any problems fitting a stock low head to vendor supplied high domed pistons., and have done several over the years.

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TWrenn
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Re: Can i run low head with domed pistons

Post by TWrenn » Mon Nov 16, 2020 3:19 pm

Welp, just learned something! So much for my gut.
Thanks Kerry


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Re: Can i run low head with domed pistons

Post by kmatt2 » Mon Nov 16, 2020 3:33 pm

It should be OK unless your new low head has been milled a lot. To check put the new head on the engine, run the new head bolts down finger tight with out the head gasket and hand crank listening for parts contact. Pull the head and look for signs of parts contact, if OK install head gasket and torque head. Note that you will need new head bolts because the high head bolts are longer and that this will not work with a after market higher compression head.


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Re: Can i run low head with domed pistons

Post by BobShirleyAtlantaTx » Mon Nov 16, 2020 5:08 pm

To check to make sure, take play-doh the kids stuff, roll it out in pancake form about 1/4 inch thick and cover the top of the piston with the piston at the full top position. Set the head on without a gasket and put a few bolts in and snug them. The thickness that the play-doh smashes to is the running clearance.

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Re: Can i run low head with domed pistons

Post by doodlebugt » Mon Nov 16, 2020 6:20 pm

Our car runs domed pistons and a milled low head (might be an 1/8th?). Done thousands of miles like that. I'd still do the plasticine thing though.
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Re: Can i run low head with domed pistons

Post by Humblej » Mon Nov 16, 2020 6:50 pm

Johnny, if you are asking if you can go from a later high head to an earlier low head you have some good responses already. The primary difference between a high head and a low head is the water jacket height, not really the combustion chamber height. If you are asking if you can go from a stock head to a high compression head, that would most likely have an interference with your high compression pistons.


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Re: Can i run low head with domed pistons

Post by JohnnyBuick » Tue Nov 17, 2020 6:51 am

The high head looks to have slight deeper chamber compared to low chamber head pictured darker on the right..the low head chamber also has raised rings where spark plugs come thru ..it could be a problem there..the high head dont have these raised rings .
I will use clay to ensure clearance.
Also the low head as you can see in pic has priming holes , is it mere bolts that screw in to plug when running engine ?

20201117_184448.jpg
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Re: Can i run low head with domed pistons

Post by Kerry » Tue Nov 17, 2020 7:41 am

The spark plug boss is over the valves, so no bother there for piston and the primer holes would be 1/8" pipe thread so try a plumber /gas supply shop for plugs for them.


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Re: Can i run low head with domed pistons

Post by JohnnyBuick » Tue Nov 17, 2020 7:52 am

Beautiful ! ..Thanks Kerry, and to everyone..!
Appreciate it.


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Re: Can i run low head with domed pistons

Post by JohnnyBuick » Tue Nov 17, 2020 7:56 am

When it comes to re-fitting the metal or copper head gasket do i need to apply any gasket sealant or is it ok to go dry ?


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Re: Can i run low head with domed pistons

Post by MWalker » Tue Nov 17, 2020 8:03 am

Use Copper-kote gasket sealer on both sides.


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Re: Can i run low head with domed pistons

Post by Kerry » Tue Nov 17, 2020 8:10 am

Pressure pack of copper seal, buy from Repco, auto barn or super cheap, or you can use brush on silver frost.
Make sure all your head bolt threads are cleaned out, scratch out the crap in the bottom of them and blow out with air. I also use never seize on the head bolt threads.

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