My original December '53 XK head needs a partial rebuild. Specifically the valve guides are shot so I’ll get new ones that’ll take stem seals, along with new valves and the required lower profile valve hardware so the seals don’t get wrecked.
Asking around, most of the older machine shops with XK engine rebuild experience within a wide radius have closed down, but there’s still one, about an hour’s drive from here. I had a phone conversation with the senior machinist about doing up the head and he seems to know his stuff - he says he’s been working on XK engines for over 40 years - but I still have concerns.
I suggested leaving the original stellite seats in place and lapping in the new valves rather than the usual 3-angle grind to avoid the potential for tappet shimming issues and the machinist responded that he routinely takes a hair off the top of the valve stems and rechamfers the edge.
Raises a red flag. How much, if any, is permissible?
Another flag. He says he doesn’t need the cams to figure out if the valves can be adjusted to spec within the standard shimming range of 0.085" - 0.110". He wants only the head with the existing valves installed as a benchmark.
This head has the original 45 degree exhaust valves and 30 degree intakes. The valves have been lapped a couple of times. Currently reading on spec 0.004"/0.006" intake/exhaust tappet clearances fitting shims ranging from 0.091" to 0.098".
I’m thinking with new valves and a minimal flat grind I should expect no shimming issues without topping the valve stems.
There’s a measurement from valve stem to the base circle of the cam, so he probably can - i would just go with lapping even if it can be ground down just a little. I lapped in new valves and the engine is happy.
Again, for a streeter, a one- or two-angle is plenty sufficient. Three-angle valve grinds are done primarily for improving flow characteristics, and on the 120, that’s really nothing more than bench raising stuff.
When I was a teenager I read about these and gave the detailed specs to the machine shop behind the counter of my local FLAPS when I delivered the heads from my 63 Corvair.
When I went to pick them up, I inquired to the counter man if they had done the three angle grind as I had requested. He yelled back to the machinist, who in turn replied “I just ground the shit out of them.” I was very impressionable, and still recall my disappointment. I had a lot to learn, as is now clear.
Valves have to cool. The inlet are not a problem with fresh air coming in. The exhaust valves get hot and the only cooling is from the time they sit on the seats. The three angle valve seat job assures that the face of the seat is concentric and the same width all the way around. This keeps one side of the valve from getting cooler than another part which helps keep it from warping. A wide seat is not needed and the narrower seat helps the valve chip away any buildup of carbon that could hold the valve open over time and warp or burn a track on the valve. This kind of job doe help flow but it also helps the job last a lot longer. All of the heads I have built get this treatment. Done properly, the 45 degree or the usual main seat area is where it always was but the surrounding area is cut down. This should not effect the valve stem height unless the seat needed a lot of cutting to straighten it out.
Thank you, DIck. If I understand correctly, you’re recommending a flat grind on the 30 degree intakes and a three-angle grind on the 45 degree exhaust valves, but not topping the valves.
The 30 degree valves can benefit from a two angle job but most stone and grinder sets don’t go below 30 degrees. As far as topping, only as necessary and only if the valve shim has plenty of space around the shoulder so as to not rest on the retainer after topping. If the valve stem is not touching the shim once all installed, it will unkey each time the valve opens with bad results.
If you have to explain this to a shop, you might want to find another shop. As far as machine work on heads, this is nothing special or unique to Jaguars.
Certainly valid advice, DIck. It’s about my comfort level after reading the horror stories in the archives about botched engine work done by inexperienced shops.
I initiated the conversation a week ago in a different thread when it looked like my only option was to have the work done by a shop with no XK engine experience, although they also said the job is nothing special. The conversation is a little different now that I have an alternative option. Both inquiries go more to addressing my lack of understanding what’s involved so that I can have an informed conversation with the machinist and be comfortable he knows what he’s doing. I now understand, thanks to you, that a three-angle grind of the exhaust valves shouldn’t affect the stem heights if the original seats are in good shape, which these are. I also now understand that topping the valves may be an option within limits, debunking my initial thinking that there should be none at all. In any case I’ll shim the new valves with the head on the bench before there’s any conversation of topping them.
The machinist said he would machine out the original 0.500” guides and install the replacement 0.502” guides after shaving them down to the original spec. I wouldn’t presume to tell him how to do the job but I’d be more comfortable understanding the best approach.
Just to be clear, it is industry standard to correct the valve length to make up for material removed from reconditioning the seat. It does not effect the metallurgy of the valve tip. Valve grinders have a specific set up for grinding the tip and chamfering. This all said… that an engine machine shop would easily say this is standard procedure and could argue it needs to be done.
As Dick points out if too much is removed the tip will fall below the retainer and the lash shim will not contact the valve stem tip. Back to the shop not understanding, they could come up with a “within spec” measurement and not know that the relationship between the valve keepers and the stem tip is a critical one.
I have had many cars come in where the tips were cut to far and the shims were slapping onto the valve. Very noisy! In this case, it is easier to come up with a patch job than pull the head, put in new seats and valves. Here is what you do if that is the case. Once the correct shim has been determined, you chamfer the edge so it sits down into the collet far enough to be in contact with the valve. If this puts the shim to far into the collet for the tappet to be in contact with the shim, then a deep reach tappet is needed. Jaguar Tappets The tappet on the bottom right of the picture in the article is the one I am referring to. Although this is a work around for a botched job, it will work and not cause any problems if done correctly. When you chamfer the shim, you will need to recheck the clearance as the shim has changed position. This is not a recommended procedure but when someone comes in with a noisy engine and there is one or two valves that were cut too much, it is a lot simpler and less expensive than pulling the head, seats, valves, gaskets, labor , etc. There, now you know one of my secrets.