Used alumna-peas to check clearances on Offy 425 heads that have been milled .020" and valve pockets cut to 450." Here's what I got .081" over the intake valve, .085" over piston center and only .040" clearance on piston's edge. With only .040" on piston edge there's no way I can mill the heads to tighten up clearance over valves and center of piston. My questions are: How much performance am I giving up on my street motor by having excessive clearance over valves and pistons? Is piston edge clearance always tighter than at the center; is this normal? Finally, when I measure net lift I get .353" on intake valve, subtract that from valve pocket depth of .450" I should have .097" over intake valve (head gasket thickness not included) yet alumna pea says .081," where is the missing .016"?
Head clearances?
- Thread starter BobM
- Start date
Walt Dupont--Me.
Well-known member
You have to measure net lift at the top edge of the valve. Every piston manufature makes the head a little differant, Some are high round dome in the middle and some are pretty flat in the middle. Walt
I've got ROSS dome pistons; I did measure net lift as you describe. Thanks, Bob
The valves approach the head surface at an angle. This is probably why you are not able to identify a direct 1-to1 relationship.
There is no advantage to adding clearance over the valves, for breathing purposes, in a street engine. However, the extra clearnce is not especially harmful, except, as you noted with regard to lower compression. Your clearance is not a significant issue.
Having a tighter clearance at the outer edge of the piston is the best condition you can hope for when using mis-matched, non-custom, components. Much better than having the tight clearance at the center of the piston.
There is no advantage to adding clearance over the valves, for breathing purposes, in a street engine. However, the extra clearnce is not especially harmful, except, as you noted with regard to lower compression. Your clearance is not a significant issue.
Having a tighter clearance at the outer edge of the piston is the best condition you can hope for when using mis-matched, non-custom, components. Much better than having the tight clearance at the center of the piston.
jdl
Well-known member
aluma peas... small balls of rolled up aluminum foil pea sized. this is a diy thing
I was hoping you would say that! thanks, Bob
I think that this question might fit into this thread, so here goes. I bought a flattie that is represented to have about 7000 miles on a rebuild. The way it looked and the fact that I talked to the guy who built it, it all seems reasonable to me. It is a 59A, Merc crank, 2 deuces, Isky 400 and Offy heads. I bought it because I am hoping that a complete engine will help me expedite a project. All this talk about Offy heads being crummy machined has me concerned so I was brain-storming a way to check clearances without pulling things apart. I hit on the idea of rolling up some aluminum foil into a stick and pushing it into the spark plug holes bending it down towards the piston. I did a couple and rotated the crank. When I pulled out "the stick" I found a crushed spot and my caliper read it to be about .125". Given that I know this is just a rough check, how would you learned flathead engine builders evaluate this finding? I did not want to get crazy forcing the foil stick in and having it break off, so I estimate that I maybe got in about an inch or so below the spark plug hole so I am only measuring an edge, I figure. Thoughts?
36tbird, just clayed the heads (alumna peas) and here's what I got .040" on the edge, .044" at midpoint between edge and center and .085" at center of piston. .125" seems a little excessive; I think anything over .100" is a little much.
I don't know how accurate your measurements would be with the method you describe but it sounds like it would work. The real experts will weigh in here and you will get a more definitive answer. About the Offenhauser head machining, Jack Schafer "FLATHEAD JACK" sells Offy and Edelbrock and runs Offy's on his race car. I think any of the after market heads probably require a little massaging.
Ol' Ron
Well-known member
What concerns me is the fact that we spend thouands of dollars on building an engine and leave the combustion chamber to guess work. For one thing a 400jr will NOt clear the head of a 425 offy, so you know some machine work has neen done.
Now here's the good part, if the valves and pistons don't hit anything the engine will run fine regardless of what the clearances are.. It just won't run great.
Now here's the good part, if the valves and pistons don't hit anything the engine will run fine regardless of what the clearances are.. It just won't run great.
I really don't know how far down I got on the piston with my foil string, so my method is a WAG. I have an engine I am building for this project, but this one came along and my hope is to expedite the project. So, I will get it going and evaluate it and I can build that follow on engine later.