My Xk120M engine is nearing the end of its 3rd rebuild in 1000 miles. The new shop appears to be doing a stellar job. I’m at the point where I need to decide which cams to use.
I would appreciate everyone’s thoughts on the choice.
Existing cams: from an XJ6, installed to quiet down excessive tappet noise (noise was actually from loose valve seats)
Original “SE” cams. Avail from. Local jag parts specialist.
John
Third rebuild in 1,000 miles?
JW,
It’s a long story.
Original build suffered a few mistakes from some pro builders.
To be fair the block is only undergoing it’s 1st rebuild in 1000 miles because the mistakes made down there were relatively minor.
The head, however, was not rebuilt with the requested specs the 1st time, so it went back for a second time. It was this this second rebuild it suffered dropped valve seats (plural) within a few hundred miles. Hence the 3rd rebuild.
The new shop is a small shop where the owner lays hands on everything, is very detail oriented, a stickler about confirming everything he does, has decades of experience rebuilding the most tricky vintage motors, and has some very knowledgable XK lovers amongst his friends/advisors. I have 100% faith he will get everything right.
Reads like a horror story. Good luck with this one.
In one of my E types I kept the original camshafts, in the other Dick Maury suggested using the later camshafts with a less tapered apex. He was right they are definitely quieter.
Allen
The choice of camshafts makes less difference that you might expect unless you had the early 5/16 lift ones. Today I think most of us use a bit more interference fit for the valve seats than usually called for. Note that the later camshafts will have a plug in the back where you want to thread in a tach drive. This can be changed but be aware of it before you put the engine together and in the car.
Service Bulletin 95 dated June '51 talks about installing 3/8" lift cams in engines that had 5/16" cams. The height of the valve guides and tappet guides was to be reduced according to drawings supplied by the factory, to give clearance for high lift cams.
The second issue of SB95 June '52 says that engines from W4012 to W4482 have already had the tappet guides and exhaust valve guides modified, and only the inlet valve guides need the 1/16" height reduction.
Engines before W4102 would need all the modifications described in the June '51 issue.
From W4483 onwards all heads have already been modified to accept the 3/8" cams.
No mention is made of Mark VII or the A serial # heads.
What has been done to head, any go fast additions to the intake tracts or valves?
I personally like the S3 XJ6 cams that it sounds like you have - is the head from an XJ6 as well?
Cams on these cars for performance variances have a number of options, but they also need to be balanced against your induction choice, head work and desired drivability.
If you have a stock head, A or otherwise, I’d plunk your XJ cams back in.
Alodmd, yes I also heard from DM that the CJ cam lobe ramps are designed to be quieter.
Jeff, it’s an xk120M engine, including block, head, and all externals (original carbs, distributor, manifolds, generator, crank damper, etc), and internal parts (OP, crank, rods, ports, valves, even the old blade style tensioner) except:
(1) Mahle 9:1 pistons
(2) slightly larger ‘C’ type exhaust valves + new properly installed seats
(3) modern lip seals on front and rear of crank.
Sounds like I’ll just keep the XJ cams as there seems to be no disadvantage wet performance, economy, and they will be quieter.
John
On yer average streeter, you can do no better.