Machining Valve Stems -- V12 E-Type

BACKGROUND. I purchased 4x V12 engine parts off of eBay a while back. The purchase included both left and right thermostat housings. The right housing still had the coolant temperature sensor in it. Bad news: it was stuck in there pretty good. After 8 days/nights of hitting it with PB Blaster, and “wiggling” on it each night with a 1/2" socket on a 1/2" drive ratchet, I finally got it to break free. The worse news: some of the aluminum threads came out with the sending unit. I took it to a local (Colorado Springs) machine shop to see if they could repair it. I wanted to rescue it cuz the eBay ad said it came off an engine with 18,000 miles. I don’t believe everything / anything I read in eBay ads but the condition of this housing is “new” – see photo which includes old sensor with aluminum still in threads.

They had the proper sized tap and were able to clean up the threads to the point I believe I will NOT need to use a heli-coil or aluminum welding to build up the material. Those options still exists if needs be.

VALVE DISCUSSION: in talking to the owner when I picked up repaired housing, I mentioned setting up the aluminum heads with overhead cams {I noticed he had a lot of aluminum engine blocks and heads in for work}. The owner said he would be able to set up the Jag heads and to bring them in with the cams and valves and such. Said not to buy the tappet shims ahead of time (I mentioned they were readily available but $6-9 apiece): he would measure the thickness of my existing shims, ID those that could be re-used, do the initial set-up, and calculate which shims I should to buy. Then finalize the head set-up. He mentioned a 3-angle valve jobs, inserting hardened valve seats, machining the mating surfaces of the heads and block; I felt good about this part of the discussion

I commented that the tips of the valve stems were not to be machined or trimmed. He replied Yes, they could be. He could remove up to 20 thousandths; after all – the stems were machined at the original factory (I don’t know this to be true). I am not a machinist, and only pretend to be a mechanic, but I recall reading here and elsewhere that Jag valve stems are not to be touched; all adjustments are to be made using only the shims.

QUESTION: So – given the above info, should I:

A) continue to use this machine shop and let him machine the valve stems (I don’t know of another in town but a friend in Denver undoubtedly knows of a good machine shop there (75 miles north)) (*)

B) it’s OK to trim off up to 20 thousandths - the admonition applies to the 6-cylinder engines; the V12 engine components were made of more modern stuff and can be machined

C) RED FLAG! Run, don’t walk, away from this shop. Go elsewhere

Curious in Colorado – Craig
(*) I am hoping to find/use a local machine shop. If this place is a no go, maybe that friend knows of a competent machinist in Denver. Otherwise I intend to crate the heads and avail myself of Dick Maury at Coventry West’s excellent services for my heads.

Hi Craig,
I purchased an XJ V12 time ago and also rebuilt the heads, replaced piston rings and big end bearings but stayed with the pistons and liners in place.
The heads were skimmed as much as only necessary, and the valve seats were reworked (not replaced). At that time the valve guides were measured (by the cnc-machine the shop told me) and one was found to be worn and replaced. I purchased new valves, valve springs and did the dis/reassembly by myself which is really not a big deal. The costs for machining the heads/seats/valve guide were only about 700USD, the material around 350 @ SNG if I remember well.
With the old shims in place i measured and calculated the correct shim thickness (don’t trust the letters) with all shims required 2,37 and 2,52mm but one with only 2,22mm. I think the thinnest would be A = 2,16 but still you could machine off some hundreds if really necessary.
I think replacing the seats or the necessity to shorten the stems are unlikely worst case scenarios.

How do your heads look like ?

I hope I did understand your post correctly - just my 2 Eurocents.