Tappet Guide Lockdown, to do or not to do?

As has been discussed many times, should the intake side be locked down or not and are early engines effected? Just got in a XK120 head for rebuild. Looks like it had just been worked on as it is rather fresh. The exhaust tappets are locked in but the intake side left alone. Two of the tappets have risen on the inlet side. Also, there really is no need for this much sealer on the cam covers…

Thanks for the post and pictures. I added the hold down kits to just the exhaust side of my three 4.2L XK engine equipped Jaguars over years thinking the inlet side was safe. I have the 3.4L XK engine from my “barn find” 1957 MK VIII in the machine shop now for a rebuild and they recommended that both sides get the hold down kits. I agreed, but felt it might be overkill. I will show them your pictures when the deliver the engine.


I just did it two days ago on the exhaust side only. Head is from a 420. I was also hesitating whether I should do it on the inlet side or not…

My intake side has one guide that isn’t fully seated anymore. The exhaust side must be much more likely to fail. There I have two with dents.

That amount of silicone is more likely to cause harm than nothing at all. My paper gaskets are not only fine but have been reused twice without as much as a smear at the half-moon seals! At least they should have put it on the outside, thin… :roll_eyes:

Just a word of caution: that excessive use of RTV isnot the RTV’s fault: it’s the fault of a sloppy mechanic.

I understand, Dick, you are not asserting the former: on here Ive seen others vilify RTV, even when applied by a conscientious, educated mechanic.

I use RTV all the time. Great invention. However, there is proper use and improper use. It is not one of those things that if some is good, more is better. Sad part is this is a re-do head done by a pro shop, both machine work and installation.

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That’s odd. We don’t really hear of tappet guide lockdowns being needed on the XK120 or other early heads; its mainly those from the '70s. I wonder if there was some poor machine work by some previous rebuilder?

I use Permatex No. 2 sparingly on clean dry surfaces and have been happy with it.

This is not the first early head I have seen with loose inlet tappet guides. It is the first one I got around to taking a picture of though. As easy and inexpensive as it is to lock them down, I do not see why anyone rebuilding a head would not lock down all 12. Early cars can get overheated also and as old as they are, unless you are the original owner, you do not know if it had been overheated in the past. It it boils down to cost, then you have the wrong kind of car. It is certainly an easy job, especially when you have the head on the bench.


Me either: cheap and easy to do.

Only from XJ6 XK engine needed the Tapped lockdown set.
Because the temperature of later XK engine was much higher.
And indeed only needed on the exhaust side.
To be save I installed these on the exhaust side of all my XK engined cars.
( XK140, XK150, S-type 3.8 )

Peter Jan

I’m always interested in learning the root cause of these sort of failures.
These tappet guides were put in cold while the head was very hot, just out of an oven, i.e. shrunk in. They shouldn’t move again unless the head was extremely hot.
If these are the original guides it would seem in this case to be caused by severe overheating; is that about right?

Overheating is the main issue. Try to get a vintage car owner trying to sell the car to admit that it had been severely overheated…

In the early models yes but I‘d think they will just stand a higher chance of moving on the later, hotter heads so it will make sense to do both sides. Even if it only made sense when overheating it would be nice to have them should that ever happen…

It didn’t occur as often as the later XKs, but we staked down a number of tappet sleeves on early (pre-70) XKs.

Back then, the method was drilling a small hole, in the valley, down to the tappet guide, then tapping the hole, then inserting s small grub screw to hold it in place.

In this video at 4 minutes 10 seconds you can see a factory worker installing valve seats and he would also do valve guides and tappet guides while the head is still hot.

I can think of a few things to share with you:

Think about whether you want to do tappet hold down kits. I do them on intake and exhaust, but others don’t believe in them or just do the exhaust side. To me, it’s cheap insurance.

Of course, with the head off, you will have a good look at the water passages and will have to decide if it’s bad enough to need welding. I recently tore down a Mk10 engine that had so much internal corrosion that I scrapped both the head and the block. It was a Florida car, and must never have seen antifreeze. Hopefully, you can get by with yours.

Be sure to inspect the seating surfaces for the oil-feed pipes and the pipes themselves. The banjo fittings often have indentations or irregularities, and it can cause a chronic leak that hard to fix. Be sure you use new crush washers - some are using a new coated style from SNG, but I haven’t tried them yet.

If you want to save your back, you can knock the center out of a couple of old spark plugs and weld on a ring for lifting. A cherry picker saves the old back as well as obviates the dropped part hitting the fender. The important word here is lifting, not forcefully trying to pull the head. It has to be loose. I also will install a head this way if I’m by myself. It allows you to be very controlled.

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whoops, wrong thread. I’ll repost.