Tappets setting, biscuit v top hat shims

PostPost by: oldchieft » Sat Nov 16, 2013 8:25 pm

Hi all
Back a long time ago when I worked on overhead cam heads I was familiar with biscuit shims.

I see someone has invented something called top hat shims.

Can anyone tell me why, and if the are there advantages to using them?

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PostPost by: rgh0 » Sat Nov 16, 2013 10:39 pm

Top hat shims are a small cap that fits over the top of the valve stem and is located by the stem itself.

The advantages are they are a little lighter and the design of the retainer has a little more freedom and can potentially be made lighter as you are not needing it to locate the shim. Both of these advantages appear to be very minor in my mind as you have no problem designing a 9000 rpm valve train without resorting to top hat shims.

The disadvantages is that they only come in a couple of thickness sizes and you need to grind them to achieve the required thickness which can be a hassle as most of us don't have a surface grinder in our garage. The bearing area on the inside face of the follower is also smaller and this could potentially cause problems especially with some of the relatively thin steel followers that are available these days.

I have played a little with them and have a stock of them and a couple of sets of retainers that use them but always in the end I have stuck to the standard biscuit shims when it has come to building an engine.

cheers
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PostPost by: oldchieft » Sat Nov 16, 2013 11:23 pm

Many thanks Rohan

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PostPost by: SJ Lambert » Tue Nov 26, 2013 12:12 am

They have been around for quite some time though, I don't know when they came into vogue, but they were being used in race motors in the mid sixties at least.......

I presume that they could well have had applications even earlier ????
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