XJ40 4.0 Head Rebuild - Bench Versus Block Tappet Gaps

I’m re-building my cylinder head and have lapped the valves. I’ve read suggestions that when the head bolts/cam retainers are torqued into the block the tappet gap will reduce 2 thou versus measuring them on the bench. (Bryan N I believe suggested this).

How accurate is this? I have to get some new shims at $5 ea. & I also don’t want a several thou surprise that makes me remove the cams again.


I’m thinking of torquing everything on the bench to full spec by using other bolts & nuts (rather than my head bolts) with fender washers to protect the surface of the head. That way I would get a good reading. Any downside?

Not sure just using fender washers would be sufficient, how much did you have to lap the valves in? Have you checked to see if the original shims will not give you sufficient clearance? When I did mine years ago I managed to use all the original shims.

Well - maybe not fender washers but wide thick washers. It was a mistake to lap. I don’t believe it needed it in hindsight. But after 240k I thought I may as well. Fine compound. Gaps were originally very close to spec. Mostly with 13-14 thou except for two 12s and another at 11 thou. No wear to talk of on shims. Perhaps .0005" on my micrometer on half of them. But now I’m getting gaps that would be all be on the lower end (12 thou ish) and a few below 12 thou if I take 2 thou off in anticipation of torque effect, so I want to check before I mess with shims. I cannot have lapped off 2 thou.

I’ll get bolts nuts and washers and torque it up on the bench.