resurfacing heads

[Brendan]

i have a 21 stud 35 motor that was rebuilt 35 years ago, it has flat top pistons, some one put heads on it for domed pistons. my friend changed the heads to flat top heads. when he changed the heads he did not get them resurfaced, should i resurface them? and could i run into other problems ,like the piston ,or valves hitting the head? please help, Brendan

[Joe AZ]

Funny, I had just the opposite problem. I had a 21 stud motor that was rebuilt in '84 and never fired. Thank God........it had domed pistons and flat top heads. The pistons were just touching the head. I found replacement '36 domed piston heads and had them checked at the machine shop. The ended up being machined to make them true. I'm sure some other, more experienced members will chime in here, but I'd suggest you take the time to clay the heads. That way, you'll be confident that you won't have any piston / valve interference issues. JMO - Joe

[Brendan]

how do you "clay heads" ? i have another friend that told me about that

[Joe AZ]

Again, I'll defer to more experienced members, but generally speaking, after the heads are resurfaced you put clay in the chambers and rotate the engine. The pistons and valves will make contact with the clay which will allow you to measure the clay thickness to determine clearance between pistons / valves and head. Joe

[keith oh]

After resurfacing. without a gasket, lay the heads on the engine, no nuts, and watch the head as you turn the engine over by hand, if the pistons do not hit the head you are OK.

[flatjack9]

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Rather than clay, use small balls of tin foil held in place with some heavy grease. Easier to measure than the clay.

[peewee2you]

Brendan, I think you should inquire at a Machine shop, one preferably that has been around long enough to have worked on Flatheads. I think the issue is if you resurface the heads and the block itself was not true? then what.. that's why I think would be good to consult an actual machinist. I know that usually plasti-guage is used to judge the clearance or thickness between rods to crank, or Crank Journal to crank. Not sure if a good tool to check for uneven surface to surface, being it is very thin which I have seen and used and is in like a string form.

[mike in las vegas]

I used play doe and a dial indicator with the gasket on. Hopefully a picture is attached.

[ironbuttjohn]

Tin foil or clay, both work. I bolt the head on(with 4 bolts) gasket in place, turn the engine over by hand, take it apart and measure what is left. You want @50 thous. No less and hopefully no more

[BrianCT]

If you have flat top pistons , there should be no problem there. If you have a stock cam , valves should be fine. If running a high lift cam , then check over valves. All this assumes that the heads are only milled a reasonable amount. Now someone has to tell us what a reasonable amount is. Certainly .050 is ok.