Being a transplanted Floridian, this issue hadn’t occurred to me until now. In South FL, anytime I set up an XK engines cam follower clearances the cold engine was always fairly close to “room temperature” - so about 70-75 degrees F. Recently I was working on a Mk2 with the old style cam profile requiring .004" intake and .006" exhaust clearance. It is the middle of winter here now and it’s been a pretty cold one in the un-heated shop. I would guess the cold engine of this Mk2 that hasn’t run in weeks couldn’t have been much more than 40 degrees F in the morning after a particularly icy night. I had a propane heater near me but it wasn’t doing much for the car itself.
This got me thinking about the effect of thermal expansion on the follower clearance. After all, .004" and .006" does not leave alot of room for mistakes. I did some research on thermal expansion coefficients that I won’t bore you with but when applied using the best numbers I could find regarding the coefficients of Silchrome and 21-4N that are the materials used for our XK’s valves, it looked like a difference of approximately 30 degrees F could change the overall length of the valve by .001"! A large amount when you only have .004" to play with.
Still not entirely convinced I devised a little experiment to check it. I set up a dial indicator on the cast iron table of my drill press and immersed a spare intake valve in an ice water batch for 5 minutes. After pulling it out of the bath I used an electronic thermometer with a thermocouple to measure the temp of the metal. 42 degrees F. Quickly placed it under the dial indicator’s probe and zeroed it out. I then took that valve and warmed it with a heat gun until the valve was about 72 degrees. Quickly placed it back under the dial indicator - sure enough it had grown .001"
Warmed it more til it was about 100 degrees - placed under the dial indicator and it had grown a total of approximately .002" I tried this multiple times to make sure nothing was moving and I wasn’t making a mistake.
Obviously there is more to this since every part of the engine and valve train would expand and contract to some degree with changes in temp, negating some of this, but it sure seems that checking cam follower clearances on a very cold engine in a very cold shop might result in inaccurate measurement that could fool you into buying shims that are as much as .001" too large to compensate. Once the engine is actually at room temperature you’d have clearances that are .001" to small. Probably not a huge problem with the later cam profile that specs .012"-.014" but in my opinion too close for comfort with the old-style cams. Something to think about…
There are definitely other factors at work and generally aluminum has a larger coefficient of thermal expansion than the steel material of the valves but there are multiple engine parts that expand and contract differently involved in the final measurement… It’s interesting… I may install a cam in a head I have laying around and measure it when it’s really cold and then again in the spring when it warms up to see what it does.
The thing that surprised me the most was just how much the valve grew within a fairly small temperature range.
When I was doing my apprenticeship for toolmaking one of the things we looked at was expansion and contraction, one of the things that came out was a piece of steel X long when stood on end shrunk by a perceptible amount. This was 55 years ago now so don’t expect me to come up with actuals
I think you’ll find this to be much ado about norhing: I set valves in a Colorado cold shop for decades, and the finishing results were all perfectly within range, which is why valve clearances are specced in a range.
Mike,
Probably 30 years ago, being curious how temperature effected valve clearance, I pulled the cam covers on a just run engine and found the valve clearances slightly, 1-1.5 thou, larger then when cold. I realize that clearances under actual running conditions may differ but I found this interesting
With the later XK engines with the modified cam profile this is true - .012"-.014" for both intake and exhaust but with the earlier engines it isn’t speced as a range. It’s .004" intake and .006" exhaust and that’s it. I’ve never seen a published range for them… which is why I’m curious about the effect of measuring these clearances in the cold.
Makes sense - Strange that Jag didn’t specify a range. I was considering just adding a thousandths to be on the safe side anyhow. Can’t hurt anything but ears if you don’t like a tiny bit more clatter.
Thanks to the crazy weather we’ve been having I’m gonna be able to satisfy my curiosity tomorrow. I installed a cam in a head I have laying around that still has the valves installed. Just threw in a random shim - the lobe I’m using was perfect at .008" today while it was warm (metal temp 58 F). I put it out on the porch to colden up tonight cause it’ll be around 32 F tomorrow morning.
I have mine set at .006" and .008" instead of standard .004" and .006" as recommended by a factory-trained Jaguar mechanic. At the time, after a fresh rebuild, the valve clearances had been set a bit too tight and the engine was very quiet when idling - when this mechanic first listened to my car right away he said “Sounds too quiet for a Jag engine” . The later .012" and .014" set-up with different cam profiles/ramps do run quieter than the earlier variety. As you say “Can’t hurt anything but ears if you don’t like a tiny bit more clatter.” Have fun setting the clearances.
The difference twixt 4/6 and 6/8 is really not that noticeable, plus it gives a little leeway for normal valve train “settling in,” and longer to a readjustment.
Mike, I’ve been running Isky cams for the past 10-15 years all set at 8 & 10 and it’s my opinion that a couple thou increase in clearance is not that noticeable as long as the clearances are all exactly the same but a valve or two with an extra 2 thou clearance jumps right out at you.
I checked it on the cold morning after a night of sitting out in quite cold weather - head measured 40 F - and contrary to what I originally thought, it seems that the clearance had actually closed up about a half a thousandth-ish. I don’t have a feeler gauge that goes up in such small increments, but my original perfect .008" seemed tighter - too tight really. .007 felt better but loose.
Just now it’s in the high 60s F, head material 68 F (20 C or ‘room temperature’) and the clearance is .008"-- probably a fraction more since it’s a little looser than my initial measurement.
I’d assume this slight cold weather shrinkage must be because of the greater coefficient of expansion for the aluminum alloy head as @Robin_O_Connor had mentioned above. Interesting… So it seems - at least on a Jag aluminum alloy head - that when measuring cam follower clearances in a quite cold shop (30 F below room temp) that the clearance shrinks slightly from where it would be at ‘room temperature’. FWIW.
Any aluminum head with some type of “overhead cam”, particularly cam bearing tower designs, will show an increase in clearance as temp rises. The biggest example I’ve seen is the Datsun Roadster 2L engine, which has rather tall cam towers with rockers below the cam. The clearance increase from cold to hot is over 2 thou. due to the cam towers becoming taller with heat, hence moving the cam up and away from the rockers. A similar effect can be imagined with the XK engine, though not as pronounced as with the Datsun engine cited.
Thanks Lee - interesting stuff. I wonder, does Datsun give a range for setting the clearance? I actually did alot of work on one of those Datsun 2000’s about 15 years ago but that was in FL and IIRC we had the engine rebuilt off site.
I found this out by setting the clearances per the manual…HOT…then checked them cold. A real bitch to check hot clearances so myself and a few other owners experimented with setting valves cold, then checking at temp…much easier and confirmed the 2 thou variance.
The phenomenon is in opposition to the more standard loosening of cold clearances, on most other engines.
People would explain to me their symptoms: car ran rough and seemed to miss cold, but then smoothed out with a little heat in the engine: PRESTO!
9 out of 10 times, it needed a valve afjustment!
