Tappet guide machining, Not a stakedown kit question


Question: I am attempting to install the cams, and shim the tappets, but my cam lobe is contacting the tappet guide. My first thought was maybe one “floated,” but further analysis reveals that it is all of them. I have been following Des Hammond’s book, Power tuning XK engines and have scoured the discussion boards and I cannot find any instance where the tops of the tappet guides need to be machined to allow for the cam lobes to clear. I am using a performance cam from Piper Cams, which does have more lift than stock, but so far I haven’t been able to find a documented solution. Piper Cams have been less than helpful.

Can anyone help with this problem?

I am rebuilding an XK engine for my Series II XJ6. Engine block and head are S III components. Machining on the head included a few new valve guides and resurfacing the deck. New performance valves, and stiffer springs have been installed. Due to the laborious nature of shimming the tappets I intended to tackle that portion myself when I encountered this problem.

So how much is the lift over standard (.375”)

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That is very odd. If I were you I would try installing a set of the stock OEM camshafts to compare the differences. Perhaps your aftermarket “performance cams” are not correct for your engine.


Just from the photo, looks to me like the base circle of the cam is larger than stock because the blank shaft is bigger in diameter than stock. This would cause the total height of the lobe to be greater than stock dimension.

@Robin_O_Connor The advertised valve lift Intake is .450" for a difference of .075" and .448" exh for a difference of .073"

@Paul_M_Novakl, I did put the stock S III cams in, no clearance issues. I will measure the performance cams this afternoon in the event there was a production issue.

Measure any tappet clearance difference between the two cams with the same shim as installed on old cam.

You can also measure the lobe base diameters for comparison.

Update: I spoke with Bill Terry (TT Engines) and received an email from Rob Beere, (Rob Beere Racing), both confirmed that this can be an issue if you are running that much lift. The solution is to machine clearance for the lobe in the tappet guide. So the short of it, the head is back at the machine shop.

@Paul_M_Novak, I installed the set of stock cams I have, no clearance issues, so I knew it had to be the cams. The cam dimensions checked out with what was ordered/advertised.

Rob Beere, offered that the way to avoid this issue is with re-profiled cams, and a smaller base circle. In order to do that, then you have to deal with oversized tappets, non-standards shims, and everything else that goes with it.

I was looking at those Piper cams for an engine I’m building - this is good info. @2Jagsyndrome can you post a pic when you get the head back and some specs as to how much was machined off the guides if/when you have it?

My thinking goes like…
if you want to stay in the standard shim range then the cam has to be manufactured/machined with a larger lobe and thus the pointy bit will hit your guide. Normally (to me) increased lift is achieved by regrinding a standard cam such that the base of the came lobe is machined off/smaller. this allows/requires a larger than standard range shim. its that shim increase that increases the valve lift. If you sketch it out you will see what I am badly describing. Piper will/can regrind standard cams for this. (I’m sure Rob Beere uses them)

So depending on what you ordered etc etc

Others may be able to explain more elequently.

Just out of interest what engine are you building? just wondered why you want so much lift.

Every decent Jag performance seller offers oversize tappets and guides. Now you know why. An XJ is not a light car and inevitably you are tuning for top end, which is a bit of a mis-match unless you attend to the gearing. What diff and transmission are you using?

You are correct, all the decent Jag performance sellers do sell the overzised tappets and guides. But that is more machining work and it seems that one you cross that line you are committed to using undercut camshafts for the rest that heads life.

As far as tuning, all of the real gains to be made in the XK engine are in the head, getting it to breath better and rev higher. I understand what you mean that it is “not a light car” with a weight of 4,114 lbs. which happens to be less than my F-type at 4,465 lbs. Also for comparison a 2002 XJR has a listed kerb weight at 4,050 lbs. and 370 HP. So I am not sure what your point is.

In my opinion the real mismatch is a de-tuned race motor in a sport-touring sedan.

I have been running the car for for years with a GM 700R transmission.

Going to wider tappets doesn’t commit you to anything. The only mismatch is large lobes that hit standard tappet guides, which is what you have and asked about and the only reason I mentioned them.

Your cam seller should have told you they were incompatible with a standard head, but wider tappets and standard or reground cams are perfectly compatible if later you wished to revert to lower tune. The only thing the engine is committed to is larger tappets, which have no real effect on anything except the potential for a wider range of cam options if you choose to go that route.

My mismatch comment was that unlike, say, bore/stroke or forced induction, high lift cams and porting tends to make the engine more peaky. Pushing the power band up the rev range in turn tends to be less useful in a high-geared three-speed luxo-barge usually driven at street speeds, If you’ve already gone to 4 speeds you’ll enjoy the tune, I guess. Have fun.