Valve clearances that change

[PhilBell]

Just measured my valve clearances (3.8, two-bolt cams) and needed to change most but not all of the shims. Put the cams back in, torqued the bearing caps and remeasured the clearances. Despite carefully measuring the old shims and the replacements, and double checking my maths, two of the inlets are still too tight (3 thou) - they had become wider as intended, but not by the 2 thou difference in shim thickness. And one of the exhausts increased from the correct 6 thou to 8, without me changing that shim!
I'm heard that it can take more than one round of this process to get the right result, but is that true? Given that we're dealing with hard measurements and simple maths, I can't see how.

[mgcjag]

Hi Phil..yes sounds like simple maths but we're talking thousands of an inch...yes you torqued the bearing caps but what torque were they befor? Did you look close at the shims. Typically a slight circular indentation forms where they sit on the valve so this needs to be factored in especially if you re use some upside down..did you keep all the followers in their original position?.. typically on your 3rd go it will be within tolerance...Steve ps getting a feeler guage in to check isnt easy and you go by feel...finally how accurate are your guages and feelers

[chrisfell]

Causes of valve gap measurememt anomalies , part 2.
2.1 Some shims can develop a small indentation from the valve stem. Annoyingly, this little indentation is just a bit smaller than the anvil on my micrometer.
2.2 More than one of the shims I found on my car's engine were a different size than indicated by the mark on the shim. I suspect a PO's mechanic had ground a shim to size rather than replace it.
2.3 I searched through a shim box of a local Jag specialist. Several of his shims were home made and obviously wedge shaped.
2.4 I don't trust shims supplied by the usuals, I always check with my own micrometer.
2.5 I once bought a micrometer on line. Big mistake, it wasn't even close. The next one I bought from a second hand tools and machine tools seller. Since then I have inherited several others....

[PhilBell]

Thanks chaps. Yes, replaced caps in correct positions and measured original and replacement shims rather than trust the markings. Also noticed the indents were smaller than my Clarke micrometer anvil. Next batch of shims should arrive tomorrow in time for round 2. Which means I'll need to have one more go after that. But once they're correct, they should be good for what, another 50,000 miles?

[Gfhug]

Phil, have you rotated the engine, assuming it’s safe to do so? Maybe that would help them settle a bit?

Geoff

[abowie]

I'm heard that it can take more than one round of this process to get the right result, but is that true?

I used to shim all the heads on our rebuilds. It usually took 3 to 4 runs through to get things spot on.

Chris's point about ignoring marked sizes is valid. I have the shim box we used. It has 1000's of used shims in it.

I always measured any selected shim before using it. They're frequently different to the markings.

[Paul bow]

As mentioned above it’s imperative you measure the shims before putting in, i remember in my first years of owning an e type doing this job with straight feeler gauges..completely useless!
You must use the right angled ones as to get the straight ones in under the cam lobe and bucket requires you to bend them slightly to get them in and gives a incorrect reading. I spent hours messing around before I was put wise!

[PhilBell]

As I mentioned in my original post, I did measure the replacement shims before fitting. I learned long ago not to trust markings or labels on shims after a frustrating round of attempts to reset the rear wheel bearing clearances. However, the need for rightangle feeler gauges is new information so I'll have a look at that. Thanks for the tip.