The original S motor from my 1954 XK120 is at a local machine shop for a rebuild. Why? After
about 1000 miles on a professional rebuild by famous Jag specialists it dropped a newly installed valve seat. The machine shop cleaned everything up and called me to say the new 9:1 Mahle pistons were hitting the head. These Pistons came from one of the most well known XK engine builders, and should be correct for this motor.
Has anyone else experienced Mahle 9:1 pistons touching their head (see photo)?
Also, don’t ever let a shop install valve seats by over boring the seat pockets for a non-interference fit, and then gluing in new seats with epoxy… The epoxy WILL FAIL, and your seats WILL fall out at the most in opportune moment.
The jag specialist cut the head multiple times. I haven’t yet measured it to see how much. The pistons were also fit way too loose on his instructions (6 thou, when it should’a been 1.5thou). I was thinking the piston was rocking enough to touch the head, but that seems unlikely.
Others on here have the expertise to comment on 9:1 pistons but I dont think they clear by very much, and I do recall others have measured their head and found it is over 9.5:1 due to cutting, but they didnt mention contact, mainly pinging
same for the piston issue, a long debate was had on this with many saying 1.5 is too tight others saying nay.
An engine rebuilder in our club who has done hundreds of Jags said he aims for 3.5 thou, but I dont know whether he was talking of Mahle
I guess you could ask the builder whether he has details of deck and block height ?
Probably doesn’t help your situation, but on a MK7 7:1 upgraded to 8:1 engine rebuild I undertook many years ago, first start-up revealed a “knocking” sound. I narrowed it down to front and top and it was only on every second revolution so I concluded it was in the timing/cam train. I got the machinist back and immediately he identified it as piston hitting head. I assume it was on the EX stroke only as on compression/power, ignition was BTDC, thus preventing the touch or tapping sound? Anyway we dropped the sump and removed the rods and pistons from the bottom without removing head or engine( cant remember if we had to drop the crank). He re-machined the piston top margins, re- installed and problem solved. I only use composite head gaskets now too.
The recommended clearance is stamped on the piston crown.
In that case Mahle use a similar alloy to 4032.
.0015 is definitely on the lower side but ok for that Mahle alloy. I do not have the drawing at hand but am sure that .0019 is the recommended.
.006 ist not used at Mahle material and therefore pistons. The Mahle similar 2618 material (MSP25) is forged in a special way and could also be used with .002 clearance.
.006 is necessary on 2618 pistons in racing and aftermarket supplier in US like JE , CP,….
I there any chance the rods were re bushed at the small end? Usually the pins are honed to size keeping the center line but some machines cut the size and could have given a longer center to center. I can’t see that making that much difference but if only one cylinder does it I would check that.
Had the same problem ( head + block machined too many times) although only at higher revs when the conrod was apparently “stretching” a few microns. I could see a tiny edge in the carbon deposit in the head.
Use a composite head gasket which is much thicker than standard gaskets. It solved all my problems. Some alternatives:
BobK, etal ,
Thanks for the many useful replies.
The head gasket came in an xk3.4 gasket kit from the usuals. The HG is single layer steel. I agree that had the pistons not been fitted incorrectly, I might solve the problem w a thick head gasket. As it is, the piston seller gave us the wrong clearances so I need a new oversized piston set, but will definitely check the piston/head clearance with some clay and order a gasket to provide better clearance. I also agree that the piston could be slightly relieved by the machine shop. How about the idea of reaming the small end bush slightly off center to gain more clearance?
Anyone recall what stock head height should be?
BTW - static compression measured at 180-185psi (warm), but I realize this is mostly due to the shape of the piston at the center and not the edges.
Here is a theory. Maybe the machine shop interpreted the piston top marking 0.15 as a dimension in mm (Mahle IS a German company). If so, 0.15mm translates to 0.006”. Seems high, but for a high expansion alloy maybe it’s not out of the question (cylinder walls look fine)
A number of friends have use these Mahles and one sent a pic of his piston crown where it has 2 markings, one is 0.040”, which I believe means it is a 40 oversize. The other marking says Sp 0.10. If this is the clearance spec in mm, then it is 0.004”
If this is the case, the 0.006 clearance is not such a major problem (noisy and cold startup), and all I need is a thick head gasket and all the other problems fixed.
Well these want 0.1 mm play
I guess it’ll be fine and believe it doesn’t have to be that exact… I flex honed until the rust was gone, then I had the block machined and they honed it again, and it doesn’t use much oil and it doesn’t have much blowby. And it doesn’t clatter.