Don Terrill’s Speed-Talk

Hi I'm new here. Was reading some posts here and was pretty impressed by the knowledge of members. I have a question that I'm hoping someone can help me with. My son and I are building a head for an engine that we built a couple years back for a little import car (a father/son build) and he wants to increase the intake valve size. We have a new intake set of the right length and guide size but the head diameter is too large for the new seats we have. Can these new valves be machined to a smaller diameter to work with smaller seats? Please don't hammer me for what might seem like a ridiculous question but I figured I'd try somewhere who would know?

i cut down valves all the time. you can use a lathe or a drill and a grinder. make sure to re face them after to get the margin right

Many valve grinders will index at 90 degrees for doing just that.

More to this if I may? Is there anything we need to do with regards to lightening the valve, back cut (not sure what that means exactly?) angles etc?? Also, are there any concerns with the hardened surface being compromised ?

A pic is worth a thousand words.

Chevyfreak.

Thank you so much for the help guys!!

When you reduce the diameter you in effect make the valve head thicker.


I've usually cut the valves' combustion face back to the original margin thickness to maintain piston to valve clearance -and to lighten it.

Momus wrote: ↑Thu Dec 24, 2020 6:46 am When you reduce the diameter you in effect make the valve head thicker.


I've usually cut the valves' combustion face back to the original margin thickness to maintain piston to valve clearance -and to lighten it.

How far up the valve do you go to do this Momus?

CnC74 wrote: ↑Thu Dec 24, 2020 11:47 am

Momus wrote: ↑Thu Dec 24, 2020 6:46 am When you reduce the diameter you in effect make the valve head thicker.


I've usually cut the valves' combustion face back to the original margin thickness to maintain piston to valve clearance -and to lighten it.

How far up the valve do you go to do this Momus?

Oh, I completely understand now. Measure beforehand and duplicate that once the valve diameter has been reduced so it won't interfere and contact the piston in the flycuts. What finish should that portion of the valve have before installation? Will the machined finish be ok?

Machine finish is fine. Valve companies such as Manley, REV, machine the margins and valve diameters in special orders. You definitely need to reduce the margin thickness back to stock after machining the head diameter. Joe-71

CnC74 wrote: ↑Thu Dec 24, 2020 2:41 pm

CnC74 wrote: ↑Thu Dec 24, 2020 11:47 am

Momus wrote: ↑Thu Dec 24, 2020 6:46 am When you reduce the diameter you in effect make the valve head thicker.


I've usually cut the valves' combustion face back to the original margin thickness to maintain piston to valve clearance -and to lighten it.

How far up the valve do you go to do this Momus?

Oh, I completely understand now. Measure beforehand and duplicate that once the valve diameter has been reduced so it won't interfere and contact the piston in the flycuts. What finish should that portion of the valve have before installation? Will the machined finish be ok?

Yes. Turned finish is plenty good.

We have LS intakes machined down to 1.875" for use in Studebaker V8s. The prototype we did on the K-W valve facer, but to do eight sets, a friend with a CNC lathe wrote a quick program to reduce the diameter, cut the three-angle and did 64 of them as fast as they could be fed into the chuck. The finish was so perfect, they couldn't be improved on with the valve facer.

We only have 4 valves to modify reducing the diameter from 39.5mm to 37.5mm. Once again this might seem lame to most of you guys who are experienced but how would we measure how much our valves must be faced off on the combustion side following the 2mm reduction in diameter to reduce their thickness as well? Also, shouldn't the facing be done first or does the sequence matter?

Measure your margin first, then cut down the diameter, then trim the face back down until the margin is the same as before.

Use the margin to your advantage. If there's plenty of valve to piston clearance, you can use it to bump the compression ratio a tiny bit if needed. If you plan on refreshening it often, the valves will last through a couple more valve jobs. If you value a lightweight valve train more, cut them down to reduce the weight.

Reduce the diameter first. Then trim up the face until the margin is the correct thickness.

For whatever reason cutting these things back makes me super nervous considering we've never did it before. Since my son is only an apprentice and worried about the consequences of a broken engine, any chance of someone here on the forum willing to cut these down to the size we're looking for as well as lighten them slightly and maybe lighten the exhaust valves too if I sent them at a reasonable price? We're in Ontario Canada