Hello all. I am starting a separate thread to get answers from the V12 gurus here.
I am in the process of purchasing valve adjusting shims to complete the assembly of my NOS cylinder heads.
As posted in my other thread "1990 Engine Parts swap" when I started to assemble the heads, all the clearances were very tight.
I had lightly lapped the old valves on the NOS heads making sure that they were sealing properly but upon assembly, I was getting between .008 and .010 valve clearances. Some were even tighter than that. I have taken the valve train apart 5 times to switch around shims but I m still coming up with the tight valve clearances. They just move around from valve to valve.
Before I purchase the required shims, I did the calculations to set all the valves at .014 rather than .012 based on some reading up I did from AJ6 engineering where Roger recommended setting the valves at 0.14. I also did the MATH to set the clearances at 0.12.
Some say I should just grind the shims to bring them into spec but I am concerned about damaging the hardened surface of the shims because some of them I would have to remove over 6 thou to bring it to spec. I have to purchase 23 shims so I will just even out the number to 24 total. The ones I need are available from Moss @ $7.99 each.
Should I go for the wider clearance of 0.14 or should I stick with 0.12.
Thanks for your input.

 

I’m no expert but about to go through the same thing myself, and have thought about a bit.

It’s a 20 year old message, but this thread has a message from Roger Bywater on the topic:

https://forums.jag-lovers.com/t/v12-...ances/128151/5

The relevant bit is “The best compromise is to run inlets at 0.012’’ to 0.014’’ and exhausts at 0.014’’ to 0.016’'. There is a power curve in the V12 Archives section of our website showing the effect.”

The curve he mentions is on the bottom of this page: V12 ARCHIVES / AJ6 Engineering

So perhaps you need to have different clearance on inlet and exhaust, not just .014 all around.

Should you go wide? Yes, because they just close up with time anyway. If it’s clattering just drive it for 20k miles until it solves itself. (That’s only slightly sarcastic, but I do put serious miles on my XJSs)

Should you just have them ground thinner? Talk to a machine shop first. Why do I think it won’t be any less than $8 each to have them ground thinner, especially as each is a varying degree (you would have to set up the machine different for each shim).

Amazing the NOS heads are only $230 and you’re about to spend $191 on shims But seriously, has anything about this been cheap to far?

Good luck!

-John