AMC heads for WBX: check exhaust valve stem clearances!

[silverbullet]

I have recently had a pair of new AMC heads that have cause untold grief and aggro because of an unexpected manufacturing fault: namely that the exhaust valves have insufficient clearance in their guides.

This is NOT an AMC valve fault since the exhaust valves were swapped for good quality, new TRW (VW) parts.

Rather that the valve guides have been either reamed too small (worn tooling) or not fully finished. I could rock them in the guide a little, but a 9.0mm reamer will not even look at the guide...

FYI a new exhaust valve stem should measure 8.9mm dia

Always check new parts, never assume that anything has been made correctly

[CJH]

Ian

I've just been checking the valves on my two new AMC heads. I've done all 4 exhaust valves and two of the inlets so far, and they all seem fine to my untrained eye. Dry they'll fall out of the guides with a faint 'pop' when I hold my finger over the top of the guide. With a film of oil on the stems they'll hold their own weight with my finger over the top.

But you've got me worried about what constitutes 'insufficient clearance' if you could rock yours a little. How can I tell if they're too tight?

[CJH]

Got it - DTI gauge. So with the the valve extended approx 17mm, like this:


I get approx 0.2mm of lateral movement (with dry stem and guide). Is this enough?

[silverbullet]

If its all new and good to the figures as per usual published sources then no need to sweat.
The duff pair I had were fitted with new ex valves which mic'd up at 0.09mm under 9mm, so call that 4 thou'ish clearance which is the usual figure for any engine.
The only true test is to use a 9.0mm machine reamer (plain shank) or a drill blank to gauge the guide, in the absence of a plug gauge.
NB: Hand reamers with a square ended shank tend to be 0.001" oversize on their marked diameter.

[CJH]

...and if I simultaneously push the end of the valve in the same direction as the seat end (so that the valve is moving parallel to the guide) then I get a measurement of about 0.1mm. This corresponds to something I read, that said measuring the 'rock' as per my photos with the valve slightly lifted out of the seat will give a reading of about twice the clearance.

So I reckon my valve clearance is about 0.1mm. Given that you were aiming to use a 9mm reamer on an 8.9mm valve stem, I'm going to breathe easy.

*E D I T - I typed this before I saw your reply. I don't have a plug gauge, but of course, a 9mm drill bit makes sense. My valves measure 8.92mm.