[BumpyDodge]

If a head is heavily milled on an OHC engine I know it will affect cam timing a little bit.(Timing belt is still the same length, but camshaft and crankshaft centerlines are slightly closer together). My questions are:

1) How much will the cam typically be retarded by head milling and block decking?
2) Is there a general rule of thumb for typical 4/I6 cylinder engines like .040" removed = XX degrees of retard?
3)Is it worth worrying about as long as I don't have valve interference or excessively high compression?

Another problem I have is that I have no record or reference to how much material was milled from the head during it's 1st rebuild. So.....

4)Is there any way to tell aside from comparing measurements with a stock head?
5)Is there any kind of reference mark typically cast into OHC heads?
6)Can you sonic-check an aluminum head?

I think I can order an extra thick head gasket to correct for milling in regard to cam timing and compression but I don't want to run into warpage problems from going too thin.

For those just curious:
The engine I am working on is a Chrysler 2.5 on it's second rebuild (definitely not "extreme 4x4" material :flipoff2: ) but the cam timing info would be general to any OHC engine and not everyone here runs pushrod engines.

 

[pishta]

you want to look up head thickness (Cylinder Head Thickness Chart, nominal size Engine Problem) and then mic your head. Sometimes there is a flat reference pad next to plug electrode in the combustion chamber that you measure the depth of off the head face, sometimes its a machined pad on the edge of the head. Milling retards camshaft timing and iirc it is a trig formula of the camshaft drive cog radius and milled distance. I had a Mazda head that was milled 1.2mm (.044) and I think it ended up being like 1/2 degree cam retarded, not substantial. This should theoretically shift the powerband higher. Take your cog and center it on a 360 compass. now turn it CCW .030 from the right edge and see how many degrees it deflected. Thats how far the cog "rolls down" the drive side of the chain when you mill the head. Cam degrees are double the crank degrees.

PS>>> just saw a Honda forum that says 1 degree (cam) retarded for every .012 off the head. that seems alot but consider 1 degree of crank is 1/2 degree at the cam. So my .044" mill is worth about -4 degrees of cam timing... Really?