Just to confirm, Martin is in Germany, but this flatness measurement of .011-.012 is being reported in inches, not millimeters?
inches. And yes, I‘ll put it on the block, and torque it down to see how the camshafts feel. should have that by friday.
And see what feeler gauges you can get in there.
Thinking about it, it is kind of odd, that we don’t have a flatness tolerance specified in the manuals. The factory drawings surely would have had something for their machine shop to work with.
We are reduced to debating it here based on our own various experiences.
The XK120 Service Manual has this phrase: “Check the bottom face of the cylinder head for truth.” What are we supposed to do, put it on the rack and squeeze it until it says, “I confess the truth.”? Oliver Cromwell would like that.
Kenneth Ball’s XK120 Autobook isn’t much better; he goes for “slight distortion” and “marked distortion”.
A bit like the days, before torque values: my 'rent’s Rolls-Royce workshop manuals read that fasteners had to be installed by “experienced fitters.”
Calibrated wrists and eyes!
The shop sent me away to mount cams and check for their free movement on the head off the block, and then with the head on the block. His reckoning is since the engine had been running with #5 valve bent it therefore should be in working order once fitted back to the block. He found the valve guides to be in good shape. Exhaust tappet guides #4 and #5 show marks from the cam lobes, so a stake down is necessary. So now to refit the cams. He could do the bending process in the oven but advised against it at this moment in time. A full quote on work needed or advised could only be done with a full cleaning and testing. Complete work would take about a week turnaround and include washing machine, hand wash/remove remaining residue, test for cracks/tolerance, new guides, seat the valves, skim asf. He did say as an example cleaning was 10, hand cleaning 20, skimming 87 euros, which to me seems a fair deal.
Once the original head is off I will not hesitate to give it to this shop to do the work.
Now out to garage.
the quote sounds reasonable to me, if the machine shop can be trusted with your precious head I would go ahead and ask himto perform a light skim, just enough to restore flatness. Making sure all guides and seats are tight and square is a good time, too. Some new valves / springs (according to careful check) may be an insurance for a long lasting repair. The block is a lot more likely to stay flat, then if the head is a bit warped for some reason chances are it’ll be difficult to keep it sealed (and metallic or MLS head gaskets will have a hard time sealing if the contact surfaces are not just right… I would use a composite gasket for a road engine, possibly with good studs for proper, reliable clamping torque).
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I went ahead and torqued (45lbft) the head without gasket to the block. New cam bearings, cams torqued 15lbft. Head is flat on the block, cams turn so easy they almost spin after giving a push. I‘ll use the head as is. Fitted hold-down grub screws exhaust side. My question now is about springs. All springs in free length are less than spec, shorter about a tenth of an inch. Mind you this engine is an interim engine not a fine rebuild. The engine has run 55 thou miles. One set of outlet valve springs is shorter by 2/10“. Will it be ok to just renew the#2 springs?
Springs short by 0.1 to 0.2 inches free length don’t make me worry as long as the spring rate is correct in the conditions as specified by shop manual and the solid length is not exceeded. Always good to check that a rogue spring is not present. And collecting several opinions is worthwhile.
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Can I check spring rate in my Garage?
You sure can! Here’s how.
Thanks Paul, that helped.