Having said the cam looks pretty good, I’m wondering about the lobe ramps. They are all exactly the same, but look to me as though the ramp is very flat. I haven’t uncovered any part numbers yet (they’re not clean enough to show yet!), and I know the mark 1 eyeball alone isn’t very scientific, but I’m used to lobe ramps with a bit more ‘curve’ than this. Opinions please - normal, or slightly odd wear pattern?
Thanks
Roger
Looks like a bog standard profile to me.
Thanks Paul, reassuring.
aree, normal, the xk profile was rather abrupt, quite possible part of the reason for the oft cited, noise=clatter, of early customer complaints, and a chane in clearances, and later add of an oil hole in the back of the cam, and finally, much later for other models the profile was changed a bit, there was a slight valve -cam time difference in the Mark 7, to the XK120. The XK120/140 5/16 and 3/8 cams are: 15-57-57-15, duration is 252, both intake and exhaust,….except the D Type 3/8 lift cam (.375) on the race D-Type cars was 30-60-60-30. Note that The Mark 7 cams are 10 50 57 15, ( this valve open/close spec is not exactly as when in service use condition, as it is measured per manual statement: “with special valve timing clearance of .010 (.25 mm ) both inlet and exhaust.) In use, Note that the XK valve clearance was at first 006 008, then 004 006, while the Mark 7 was always 004 / 006.
Nothing out sorts here. Standard flank profile.
The flat flank really does not do much. The large diameter of the XK tappet lets the cam nose come into contact much quicker than on most other engines where the tappets are smaller in diameter (American V8’s, for instance)
Thanks chaps. I don’t think I’ll be replacing these cams then, which is a bit of a relief given the problems I’ve encountered with replacement camshafts in my Ford V8s in recent years. Material quality has not improved, in my experience.
There is a useful cam and tappet animation in this link. The illustrated design is quite similar to the XK valve train, with the notable exception of the tappet diameter. You can see that by only making the tappet diameter larger you can open the valve earlier and close it later without changing the actual cam profile. Of course, the the area under the curve is where the performance is developed.
http://www.enginebasics.com/Engine%20Basics%20Root%20Folder/Basic%20Camshaft%20Understanding.html
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you do plan on checking the clearances and adjusting as needed? Should you need a detailed description of that process, in Saloon archives, PSalt wrote it up, I also have it with some more,can send to you, Decision to be made if you want to use 004 006, or 006 008, I opted for the 006 008. Nick
Yes Nick, I’ll be setting the cam timing from first principles once the heads have been machined - new guides, recut/replace seats, etc. Thanks, though, any extra info would be welcome.