XK120: My pistons protrude 0.045" above the block -- what to do?

Hello All –

I received all my bits and pieces back from the machinist some months ago and am now burning up some vacation time to build it back up. In utilizing Ray Livingston’s compression ratio calculator, I mentioned to him that I had to go negative with the piston-to-block deck clearance. That got his attention as he was quick to reply that the pistons normally stop below the top of the block when arriving at TDC. I’m really puzzled by this and am wondering if others have run in to this before? The pistons are new Mahle 0.020" oversize sold specific for the Jag 3.4L. I remember comparing them to the originals and they sure looked the same before discarding the latter in the scrap bin. As for the cylinder block: I know it hasn’t been shaved in the past other than my machinist’s light clean up as the letter markings for the original piston sizes are all still very legible and deep. As for the crank, my machinist said it had so little wear all he had to do was a light polish – so, nothing too involved there. The cylinder head had to have 0.016 shaved off the bottom as it had warped in the distant past .

So now I have pistons that protrude above the block and a head with chambers that are even closer to the block than they normally would be. What would be the calculation for purchasing a head gasket while still allowing for adequate valve clearance while still trying to maintain an 8:1 compression ratio? And at what point does a thicker head gasket make for an upper timing chain that won’t fit the sprockets? One of the local suppliers has an off-the-shelf Cometic at 0.074" available but I’m wondering if even that may not be enough. Ray suggested procuring shims and modeling clay followed by rotating the engine then checking clearances, but that’s putting me way out of my comfort zone here.

Here are my measurements so far:
*Cylinder head chambers: 99.3cc
*Piston dome: 17.1cc
*Piston-to-deck clearance: -0.045"
*Cylinder bore: 3.288"
*Stroke: 4.1732

Any thoughts going forward would be appreciated as my head is swimming in numbers and I’m also confused as to why the different compression ratio calculators found on the 'net can yield such different results.

First I would get the specs for original pistons and go from there, attempt to return for exchange or refund if they are made wrong or fix what you have. Depending on how much material there is above the top piston ring, it shouldn’t be a big deal getting these pistons machined to fit right. Most pistons are pretty thick on the top

negative piston decking height is what you got: they protrude above the block.

I quickly ran the compression calculation with your numbers, and find just about 8:1 (using a 1.3mm compressed height gasket), so that would be what you asked for, too.

I noted your chambers are still large despite the 0.4mm skim (did you actually measure them, that is with the valve in place)

A thick gasket like .074 would get you a compression ratio to around 7.7:1
You may want to double check the installed height of the initially intended head gasket, but if the machinist is good I would believe all should fit (installed height probably above .045), and you don’t need to decrease your intended compression ratio (unless there has been other considerations to account for since the initial specs definition).
A professionnal machinist should have no problem explaining the choices made for your build : do not hesitate to talk to him if anything seems unclear.

a valve clearance check (plasticine) could be reassuring, but if you want to save a blank assembly you probably can check the numbers and calculation carefully, as it is a stock engine rather than a race one (high lift, wild durations etc).

ps: thinner MLS Cometic (which I suppose is what you’re referring to) I’ve found difficult to seal, they are quite demanding on the sealing surface roughness (and flatness of course), maybe not so much for the thicker versions… I tend to stick to composite head gaskets if I can.

My pistons are also above the deck. This is using original pistons, crank and no skimming of head or block, tin head gasket . I think all of the components are original but not 100℅ of the pistons.
It used to run before so the height didn’t concern me. I didn’t measure but I guess 1/2mm

Nic –

I measured my chambers with the valves, including spark plug, in place. They all averaged 99.3cc +/- 0.2cc. I believe my measurements to be spot on as the small opening on the bottom of the burette was exceedingly slow to dispense fluid. Each chamber took an average of 20 minutes to measure out.

Composite gaskets. I ordered my oversize piston set from Barratt (last fall after I’d dropped the engine parts at the machinist’s and they in turn called back saying to order 0.020 oversize pistons). When I received my new pistons, included was the pictured gasket shown below. I just assumed it was a standard size item. It’s marked 1.8mm which works out to 0.070". And when I enter the gasket part number on Barratt’s site, up comes the listing for the 0.020 Mahle piston set I had ordered. So it looks like they know the pistons stand proud of the block upon assembly and as a result this gasket has to be installed with them in order to compensate for that? I should call the US branch I ordered from in order to learn more. So if this gasket seems to compensate for the height of the pistons will it also compensate for my head having had 0.016" shaved off it’s bottom? I can’t physically confirm because in the meantime, I’m fabricating a new timing chain tensioner bottom bracket of steel as the old one is of aluminum and had failed. Thus, I can’t test valve/ piston clearances until I get the chains on. Wow. Things were so much simpler when I was 17 and I was able to throw the engine back together on a '59 MGA within three days after getting things back from the machinist.

Jim, I’ve been following your postings with interest as you and I are currently roughly at the same point in our restorations. Especially your recent posts regarding timing chain fitment. Your 1/2mm guess on piston height above the surface leaves me with some reassurance at least that my assembly isn’t some fluke one-off. Have you tried calculating the compression ratio for yours?

I haven’t yet put the head on my S1 4.2 engine so I measured mine. I see .026-.029 below the deck surface of the block. The block has been decked recently but I can’t say how much.

No I haven’t measured anything. I quite like nothing being changed and at factory spec , albeit at bit more worn.
I spoke to the guy that owned it in the 70’s . He replaced his 140 with a Corvette and was disappointed with the performance after the Jag.
That sounds good enough for me!

Corvette’s are just an overpriced piece of plastic anyway.

Art, this might be one of those peculiarities associated with low production early models long since worked out by the time your engine came along. Count yourself lucky.


If this is any help I have recently re-pistoned a 1965 3.4 ltr engine with Mahle pistons and 1.8 mm gasket from Barratts. The bore and crank are still standard.
The head had 0.030 inch removed and I know it had been previously been reground, how much I am unsure. The block face is original and has never been ground. The pistons protrude above the block by 0.0275 inch.
The engine has nearly completed 1000 miles with no problems

Since you’ve done it yourself (opposed to using the book or having it measured by someone else) your chamber measurement should be good, within 1cc at the most , so that’s not a source of uncertainty. I was just surprised it to be on the large side though the head has been skimmed (granted, only a little : 0.4mm would reduce chamber volume just a bit over 2cc), it may have been volume balanced after skimming to get there (a good thing if it has).
My calculation with the 1.8mm gasket gives about 7.8:1 CR (to get 8:1 CR you’d need fo fit a 1.3mm compressed height head gasket).

I’m not 100% sure just by the look of it but the gasket does not look like a Cometic (MLS for multi layer stainless steel - they are very good, but need precise tolerancing), so I would think you should not need to take specific care for it to seal, just regular straigtness (as your machine shop would have checked or adjusted) and cleanliness upon reassembly. 1.8mm (I assume compressed height) will compensate for head been skimmed, and some of the piston negative backspace. I think you should be allright and spot on with the specs you had intended.

I would not fret over valve touching the dome either if your camshafts are stock.

Only thing I would wonder about is the choice of a 8:1 piston with as a result a crwon standing proud a bit (they must have selected the compression height just so according to your specs), while a 9:1 with a larger dome, if available, may have resulted in a lower piston crown for the same compression ratio. In any case, again call your machine shop in case of any doubt, but all seems fine as seen from here (granted, I’m a bit far…).

good luck!

Okay, that’s also reassuring – thanks. I remember the machinist saying that I could have still kept the original size pistons since they were still technically within limits – but barely so. He said it would have been a smoker though. My '37 Sunbeam motorcycle already does enough of that so that’s why I opted for the oversize.

I looked at this pretty closely about 10 years ago when I was in the process of swapping on the straight port head. I don’t think that is a problem on a 3.4L. Keep in mind that the combustion chamber is hemispherical, not wedge, so no fixed physical overlap. Any contact would be with an open valve, not the head itself. The combustion chamber location, center to center distance and volume remained constant (about 100cc, I recall) throughout the production of the XK engines, all the way out to the last 4.2L XJ6. I would note that the top edge of the piston is chamfered to clear the edge of the chamber. You could not get away with this on a 4.2L where the center to center distance between the cylinder bores is different than that of the combustion chambers.

At this moment I’ll stick with the gasket provided. It’s not an MLS Cometic. It looks to be some sort of composite sandwich of pressed fiber something with a thin sheet of unknown metal on the inside. Stainless steel surrounds the cylinder bores. I think it should do. Except for longevity modifications, the car is being kept largely stock.

Yes, Ray mentioned not having to worry about cylinder offset as well. This didn’t register with me at the moment until I remembered once reading somewhere about that same thing with later engines. This is only my second Jaguar. And I never had to be so hands-on with this one like the XJ6 that preceded it.