Shim-over-bucket Miata - shims all too thick after head work!

I purchased a '99 with no compression in one cylinder; turns out an exhaust valve had a hole in it :eek:

My brother has been doing all the work on this one (it will be his daily), took it to a machine shop he trusts, they did a port/polish and re-seat/3-way grind? (showing my ignorance here)

Anyway, the final result was movement of the valves such that the shims we have will be too thick to use for any of them. Was this a mistake? Probably. Too late now.

Far as I can tell, we have the following options:

1. Buy thin valve shim, use it to measure required shim thicknessses for all 16 cylinders, order all from cylinderheadsupply @ $20/4 for a grand total of $120 (plus shipping)

2. Grind the valve stem tip to match factory clearances? Head is already disassembled, this could be an option if it won't cause issues with the valve stem hardness. Not sure at this time how much material we would need to take off.

3. Use surface grinder (I have one at work I might be able to use) to grind existing shims to proper spec. Again, not sure how this would affect surface hardness.

4. Convert to SUB style for more money up-front ($200-$220 plus valve lash caps), but future-proofing for different cams or higher RPM's, plus less reciprocating weight.

5. Other?

Can anyone advise on which method is best moving forward? I think I've read near every thread I can find on every Miata forum about this subject, before anyone asks.

That said, I'm still an engine-building newbie. Thanks in advance for any advice!

AIUI, typically one grinds the valve stems.

The tips of the valves need to be ground roughly the same amount as was removed from the seat and face of the valve. That only gets you in the neighborhood, though. You need to find a guy with a selection of shims, buy your own, or give the head to someone with the ability to adjust them properly.

Thanks y'all. Will talk to machinist this week.

Why bother grinding the valve tips? You will still need new adjustment shims.

In reply to Chas_H:

Because you can only go so thin with the shim. Its prudent to keep valve stem heights in a certain range.

In reply to Streetwiseguy:

I understand that. But we don't know what the clearance is. Grinding valve stems until there is clearance is tedious and the OP might not have the equipment to do it. Valve stems can be ground if it's needed. This is something the shop should have addressed when doing the work. The bottom line is grind, perhaps more than once, one shim or 16 valve stems.

The tips should have been ground when the valve work was done but it's less critical on an OHC engine like the Miata's since the basic geometry isn't messed up by stem height. What I've done in the past is put the head together without shims and measure the clearance to determine what's needed.

Often if the valves will sit so deep in the valve pocket that clearance will be n issue they replace the valve seats....

I agree that grinding stem tips shouldn't cause issues as there is no side to side component to the motion over the tip as there is in a rocker engine, so unless the stem material is pretty darned soft and mushrooms, it shouldn't affect it.

Chas_H wrote: In reply to Streetwiseguy: I understand that. But we don't know what the clearance is. Grinding valve stems until there is clearance is tedious and the OP might not have the equipment to do it. Valve stems can be ground if it's needed. This is something the shop should have addressed when doing the work. The bottom line is grind, perhaps more than once, one shim or 16 valve stems.

Grinding tips is a standard part of the machining of a cylinder head, to refresh the tip. OHV engines are generally worn due to the rockers, OHC not so much, depending on the design. My advice, though, was to buy shims, borrow shims, or sublet the job. I was not suggesting he try to grind the tips to spec.