1954XK120M, 9:1 Mahle pistons contacting the head

The original S motor from my 1954 XK120 is at a local machine shop for a rebuild. Why? I had it professionally rebuilt by some famous Jag specialists, and after 1000 miles it dropped a valve seat.
The new machine shop (not a Jag specialist, whew), cleaned everything up and called me to say the 9:1 Mahle pistons were hitting the head. This is happening on all 6 cylinders. These pistons came from one of the most well known XK engine builders in the country, and AFAIK are correct for this motor. The block was machined and pistons installed per that expert’s specifications (6 thou piston/bore clearance). FWIW, these are supposedly the Mahle’s sold by the regular Jaguar parts places.

Has anyone else experienced Mahle 9:1 pistons touching their head (see photo with contact area noted by red arrow)?

No answer regarding the pistons hitting the head but it appears to be only <>110deg?

Hello John,
Mahle controls the drawing- dimension 100% . The tolerance on the compression height is +0.05/-0.10 mm.
There is no chance that a part not to drawing will be sold especially 6 of them.
Either the block or head were skimmed too far or the cylinder bore/clearance was done wrong that the pistons could rock because of bigger side clearance . On the other hand there is bigger noise during warm up at least.
I personally tip on skimming. Could be corrected by a thicker head gasket or a good turning machinist removes material on the piston in that area only 0.1-0.3mm (without destroying the piston profile!).
Best regards Thomas

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did you use a MLS head gasket ?
if so did you get the right thickness

Yes, I think I have both problems on this motor.
Need a complete redo because of excessive piston clearance, dropped valve seats, cracked head (value guides improperly installed?), etc.

Thanks for the good suggestions.

Update: I’ve ordered new 0.040” over Mahle 9:1 pistons to replace the ones coming out (they have about 1000 miles on them). This time I will have clearance to the head checked with clay or plastigauge and order thicker head gasket to provide the needed clearance.

The machine shop I’m using says the proper piston/bore clearance is about 1.5 thousands (inch) per the factory manual. I believe Mahle recommends 0.1mm (0.0039”), as is stamped on the piston crown. My gut says the piston maker knows the thermal expansion of their alloy.

Which piston/bore number would you use?
3.9 thou or 1.5 thou?


Oh, and here is a pic of the piston (all 6 show this at least part way around the crown). Slone of you asked what it sounded like at startup. Several Jaguar expert shops checked it out and said the noise was just typical cold startup tappet noise. I thought it was similar to other xk120’s with poorly setup valve clearances.

After this expensive lesson, I stopped trusting any shop, no matter how stellar their reputation within our community. I’ve gone back to my old habit of doing and checking everything myself. Yes I’ve made the occasional mistake, but nothing as bad or as expensive as what I’ve experience with renown Jaguar experts.



How much damage to the head? Any pics?

I would always go with the piston manufacturers specs on clearances, it all depends on the alloy used.

Just scrolled up and saw the head photo. Sorry.

Definitely what Mahle says. Trust the manufacturer.

This is David‘s 4.2 engine for his S1 XJ6, showing new Mahle pistons stamped with dimension and clearance spec. I would go with the manufacture‘s data.

Was there any valve seats replaced during the last rebuild?

Very sorry to hear about this. Quite disheartening.
A couple of years back, I rebuilt my own XK engine. Bought Mahle crankshaft bearings from a top supplier in the US. I had to get 3 separate boxes to finally make a full set of bearings in acceptable condition…

Watch this: Opening the crankshaft bearings and finding surprises. - YouTube

Is the head original to the engine?

None of those slight marks would’ve bothered me in the least: however, the big divot in the center of that one bearing certainly would’ve!


yes original head & block.
Yes, I paid for the exhaust valve seats to be replaced.

When my current shop cleaned up the head, they found other seats loose, and they subsequently discovered the offending jag specialist epoxied them in place.

Even the best shop in the world makes mistakes, and we who own old cars need to be understanding about this possibility. In this case, I thing the jag specialist was simply too busy to check the work of their subcontracted machinist, and this error snuck out the door.

The silver lining is that the dropped seat didn’t break a valve off and ruin the motor. It could have been very much worse.

Currently, the new shop has carefully welded up the old seat bores plus some cracked valve guide areas, and is cutting the head for new seats to be installed with a proper interference fit. I’m sure there will be some head warping from large amount of tig welding, but I’m ok needing to use a thicker HG (Cometic) in order to keep the original motor together.

It’s a good thing we do this without an expectation of a positive return in our $.


They epoxied the valve seats in?


Don’t you know JB weld is the go to repair😇

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I have just see the pic of the piston it looks like the markings pitting on the top side of the piston go down as far as the first ring
if that is the case how did the piston not bend the rod or smash the head ?
my guess is that what your heard was pinking because the ignition timing was wrong
I have had the same sort of marks on my turbo Volvo pistons with to much boost

Right, probably when the piston touched the head, some aluminum debris resulted, which scratched the piston a bit. I suppose it could also be bits of valve or seat.

I bit the bullet and bought a new set of the same pistons at 40 over, which will enable the machine shop to close up the piston cylinder gap (currently at 4-5 thou).