Fun with tappet guides

XK-Engine
1

@Wiggles very kindly gifted me an XJ head to replace my scorched, cracked, and pitted original which I deemed beyond saving.

In tearing it apart I found that one of the exhaust tappet guides was thoroughly bashed all around it’s circumference. Interestingly it was now all the way in and not contacting anything, and the cam shows no signs of contact. I imagine it popped out at one time and the previous repair involved bashing it back in without remorse and possibly replacing the cam.

Having my old head with intact guides I decided to rescue one of those and use it to replace the wayward one. It was actually too tight for the bucket, so after a lot of careful sanding the guide ID and checking for interference I had a perfect fit that slid and rotated easily with no discernible play.

Now I wanted to fit it into my replacement head, and to my disappointment it just slides in the hole without any resistance. Of course I should have checked it first (yeah hindsight), but measuring the bashed up guide reveals it is .002" oversize.

A bodge fix such as manually “knurling” the OD of the now endeared-to-me standard guide (I know, I shouldn’t have named it, sorry George) came to mind, but then of course the aluminum of the not-yet-as-endeared-to-me replacement head will be permanently scarred.

So I figure I can attempt to do the following.

  1. Get an .010" oversize guide from SNG
  2. carefully hone the head for a .002" interference fit using a brake hone or somesuch
  3. carefully bash the new guide home
  4. carefully hone the ID of the new guide for the proper fit of the bucket.
  5. stake all of the exhaust tappet guides down for better sleeping at night.

I feel like #2 is the most iffy. The hole is not particularly deep to allow for self-alignment of the hone, and being aluminum is may end up oversize in seconds. Using a friends milling machine could be an option, but of course quarantine and whatnot will mean a delay in that. This seems like overkill for sending to the machine shop imo, but maybe I’m being naive.

What do folks think, I feel like using the standard tappet with no interference fit isn’t smart, even with the stakedowns, and bodging it doesn’t seem great though it might be fine. Fixing it the “right way” seems like a pain, but this is nothing new.

Thanks!

2

Id go for #2: heat the head on a stove, and dunk the guide in a plastic can of dry ice and isopropyl alcohol: it should drop right in.

3

The dry ice suggestion is a good one. Would be interesting to see how much that changes the OD of the guide. Thanks!

4

I guess another thought would be to very carefully sand/grind the OD of the new oversize bucket, but lacking a lathe I imagine that’s impractical to do precisely enough.

5

Unless you have a lathe…don’t.

6

Will the head fit in a 30" wall oven?

7

Or would BBQ with a hood be better? At least it would be outside for any fumes… if the neighbours ask what your cooking you can say it’s braised head of cat…

3 Likes
8

I’ve used a standard BBQ grate. Flavers the hot dogs, next use…:wink:

9

Heh, definitely not, even my gas BBQ is too short. Luckily I have a rectangular gas fire pit that should work with some sort of stand to put the head on. Also doing it outside is better for my marriage, though my wife is actually very tolerant, I’m not sure roasting my head on the stove would pass muster.

10

Yours…or the Jag’s?

:laughing:

11

:fire: :hot_face: hahahahaha ouch!

1 Like
12

I have seen EXACTLY the same thing on a S3XJ6 head. Not a lick of damage to the cam.

I’ve got a half dozen or so new tappet guides, you are more than welcome to one. They do have to be honed to suit the tappet.

I bought the 4.2L engine with the beat up guide from the local Jaguar dealer. They had a reception for the local Jaguar club for the launch of the XK8 (1996?). The engine was sitting in the corner of the service department. A S3XJ6 they had traded for had a ticking noise, and rather than fix it, they replaced the engine. Anyone who wanted the pull-out could write a dollar amount on a slip of paper and the high bid at the end of the night took it home. I got it for $100.

3 Likes
13

Ditto: never saw any damage to the lobes.

14

I measure the tappet OD as as 1.3735", the guide ID as 1.3545" and the OD as 1.644" (digital caliper, ymmv)

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Thanks Mike! You all are a generous bunch! Given the pile of spare parts I am accumulating I’m sure I’ll be able to lend a favor to another pool soul someday.

I love the story of your engine acquisition, being at the right place and knowing what you are looking at definitely has its advantages.

I imagine the shop manager just wanted that lump the heck out of there.

16

This bashed one measures 1.627”, the standard one 1.625”, the hole in the head (not the one on my shoulders) comes in around 1.626”. My tappet is also right around 1.373” OD, I also have a selection of others care of @Wiggles :smile:

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17

Man, that is about the worst, bashed-up tappet guide I’ve ever seen!

18

The one that passed through my hands was just as bad. These tappet guides look like they are sintered or powdered metal, very porous. Obviously, if a camshaft lobe can beat the snot out of it without being damaged, they must be pretty soft.

1 Like
19

Unless they crack, and I heard they can!
The cams take absolutely no damage. When it’s back together, a stakedown kit or grub screws from the valley are highly advised.
I have one badly smashed (interesting how they rotate, eh?) and on another head have seen two or so with slight nicks. It doesn’t have to be a catalysed engine for it to happen.

20

The cam lobes are very slightly off-center relative to the tappet. That very slight offset is just enough to impart a slight rotation to the tappet, enough that any wear is evenly distributed across the entire face of the tappet. I believe the guides are sintered iron, very soft and porous.

1 Like