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Low head
Posted: Thu Oct 28, 2021 8:41 pm
by BarrettR
Can high compression pistons be used with a low head or will the pistons hit the head?
Re: Low head
Posted: Thu Oct 28, 2021 8:50 pm
by Joe Bell
I asked that once and they told me yes.
Re: Low head
Posted: Thu Oct 28, 2021 10:12 pm
by Wayne Sheldon
Generally yes. However, some higher compression pistons are higher than others. There were variations in the original head castings for several reasons. Couple those with some people mill more off of heads than is really wise in the futile belief that that extra 1/8 inch really makes a difference?
So check for fit BEFORE bolting the head down. Place the head on the block without any gasket, and make sure pistons and head do not hit! If it clears fine without a gasket, it should be okay bolted tight with the gasket.
I did see one once that someone did not check it first? Turned out one of the pistons near the center hit the head when they tried to run the starter. Ruined the center main. And probably the crankshaft?
Re: Low head
Posted: Thu Oct 28, 2021 10:41 pm
by Kerry
I've fitted several sets to early T engines with no issues, always found ample clearance in the combustion chamber.
Re: Low head
Posted: Fri Oct 29, 2021 8:56 am
by speedytinc
Wayne Sheldon wrote: ↑Thu Oct 28, 2021 10:12 pm
Generally yes. However, some higher compression pistons are higher than others. There were variations in the original head castings for several reasons. Couple those with some people mill more off of heads than is really wise in the futile belief that that extra 1/8 inch really makes a difference?
So check for fit BEFORE bolting the head down. Place the head on the block without any gasket, and make sure pistons and head do not hit! If it clears fine without a gasket, it should be okay bolted tight with the gasket.
I did see one once that someone did not check it first? Turned out one of the pistons near the center hit the head when they tried to run the starter. Ruined the center main. And probably the crankshaft?
Agree with all of this.
I did this to keep the stock appearance on a 14. Milled an extra .100" off the head. More is not always better. Pistons needed trimming on the non valve side. 95# compression. It runs great, but not as well as I expected for the extra compression. It the law of diminishing returns with compression. Going from 4:1 to 6:1 is a hugely,noticable improvement. going from 6-7, not much. Would not do it this way again. IMO The extra milling didnt seem to gain that much?? The size & inefficiency of the valve chamber is the issue IMO. IMO stock pistons & a Prus head is a better way to go for performance/efficiency IF you dont NEED the low head look.
Re: Low head
Posted: Fri Oct 29, 2021 1:49 pm
by BarrettR
Where is the Prus head available?
Re: Low head
Posted: Fri Oct 29, 2021 2:01 pm
by speedytinc
BarrettR wrote: ↑Fri Oct 29, 2021 1:49 pm
Where is the Prus head available?
Advertised in the vintage ford. IMO best head available. Not as high compression as claimed. Dont let that scare you off it.
There have been recent discussions regarding high compression heads here. Search Prus.