Valve lash DOHC

[TomMull]

I'm working on a '70 Lotus plus 2 but I posted this in the British Other forum since I think it would apply to anything with an overhead cam or cams with lash adjusted with shims. I came up with this after getting closer and closer to specs after several tries (removing and installing cams each try) but still a few thou off.

I’ve finally finished setting the valve lash on my Lotus Plus 2 head. I had a local machinist install new valves, guides and seats and I can find no fault with the work. I decided to do the valve clearances myself, since that’s what an old retired guy like me likes to do.

It turned out to be a bit of a challenge. In order to establish a base line, I needed to install some shims. I used the old ones as that’s what I had pretty much at random. They ranged from .020 to .080 (no idea where the PO got the thin ones). Also found the seats on the cam followers and valve stems ground off, all replaced of course.

This meant I had a wide range of shim thicknesses from which to calculate proper shim size and some of the very wide gaps that were difficult to measure accurately with my feeler gauge.

The solution was to buy a couple of sets (4) shims to use as gauges. I bought .060 and .090 although .060 would suffice and don’t mix them. This eliminated one variable and made the measurements and calculations much simpler. Perhaps others have done this but I haven't found it in the lit.

For what it's worth,

Tom

 

[billspohn]

When I built a hybrid race engine for my MG race car using a 1950 cc 5 main MGB block with an MGA Twin Cam head grafted on, I couldn't easily source original style flat shims so I used the little cup style shims that early Alfa used (IIRC there was a change in stem diameter between early and later Alfa, so you need to choose which ones suit).

Most garages that service Alfas have shim sets and will sell you the sizes you need without you needing to buy a whole set.

 

[TomMull]

Very interesting engine you built. Must have been a challenge to get the cams timed. A friend of mine, who was a car dealer, had a twin cam MGA back in the early 60s. I think it's the only one I've ever seen.

 

[billspohn]

Very interesting engine you built. Must have been a challenge to get the cams timed. A friend of mine, who was a car dealer, had a twin cam MGA back in the early 60s. I think it's the only one I've ever seen.

Yes, it had some challenges but it helped that the DOHC engine had been based on the pushrod design. I used late press fit wrist pin connecting rods (custom racing con rods were not yet generally available for MGBs at the time) and had forged pistons made by Ross Racing. Lots of calculations to get the pistons to suit 12:1 compression that I wanted, and the cam profiles were something I patterned after the DOHC Ford engines - I used a Cosworth L1 race cam that was then being used in the BDA engines and it worked out really well. And I was lucky enough to have a factory 5 main race crankshaft - cross drilled etc. to use.

It was great fun once it was all set up properly - could blow past the big Healeys and Jag XK 150S on the straights! Ran a 3 synchro early MGB trans with an OD (used only in 4th gear) and had a close ratio factory gear set. Had a few excellent races including one with bathtub Porsche at Portland that I couldn't figure out - he was running right with me, which didn't usually happen. Turned out to be a rare factory 4 cam engines - 356 Carrera GT! (They now sell for around a half million bucks with a peak of over a million!)