valve clearance and shimming

[paddy]

My approach to eliminating any risk of valve-piston or valve-valve contact:

- set the engine to TDC and then turn the back crank by 90 degrees; the pistons are all now half way down the bores;
- remove both cams;
- now with each in turn, you can play all day with no risk of contact - adjust, measure, remove, reshim, refit, etc;

- to put it back together:
- fit both cams in the correct TDC position (sprockets loosely fitted);
- turn crank forward to TDC (don't go the long way round);
- fit chain, tension and secure.

Maybe it's overkill. If you keep the timing chain in tension (eg with a bungee as someone suggested) you can usually get away without having to re-time the ignition but you should always check. I didn't think you could safely do it with the engine at TDC.

Paddy

[ecamiel]

Alfa shims are small diameter cap shaped. Much lighter in a critical area. I believe all the racers use them and they are easy to find.
Eric
64 S1 Hart

[Esprit2]

My approach to eliminating any risk of valve-piston or valve-valve contact:
- set the engine to TDC and then turn the back crank by 90 degrees; the pistons are all now half way down the bores;

Turning the crank back 90 degrees is standard Lotus procedure in the later 907-910 manuals. The Twin Cam manuals are a little over-economical in their use of words and could have benefited from a few additional steps in many procedures.

Maybe it's overkill. (Snip)... I didn't think you could safely do it with the engine at TDC.

90 degrees puts all pistons halfway down their bores, which maximizes the distance between them and the valves. However, since valve lift with even the wildest of TC cams is under 0.500", there's really no need to go that far. 90 degrees is just the simple, safe default. It may be overkill, but a little overkill isn't a bad thing.

Do follow correct procedures. But in addition, "always" use a light touch. Contact doesn't automatically bend valves. Force bends valves. Whether you're turning the crank, or a cam, approach it like it's attached to something expensive you don't want to pay to replace, and turn it with a little mechanical empathy. If you feel contact, stop immediately. "Clunk" is not good, but it doesn't necessarily do damage. Continuing to pull after the clunk does the damage. Use a light touch and pay attention to the feedback you get from the engine.

Regards,
Tim Engel

[rgh0]

Alfa shims are small diameter cap shaped. Much lighter in a critical area. I believe all the racers use them and they are easy to find.
Eric
64 S1 Hart

Yes quite a few racers use them coz the suppliers sell them as a "racer mod" but a pain in the a**s as you have to grind them to get the correct clearance as they typically come in limited thickness sizes. If you weigh all the reciprocating parts in the valve train the shim weight and retainer is insignificant compared to the valve head, valve spring and bucket (even the light weight steel buckets) and the weight saving going to the tophat style shim or titanium retainers or smaller diameter valve stems is not really signficant or worth pursuing IMHO even for a 9000rpm race motor.

Just my opinion around a bit of dynamics
cheers
Rohan