Having to have some head work done after a not very good engine rebuild by so called experts and have been asked why the original engine builder has used top hat shims instead of standard. We went down the fast road option for the rebuild but I have no idea, can anybody help with an answer.
Thanks

 

There is not a problem as such with top hat shims, they are easier to get hold of from a standard engine catalogue, the Jag shims are 17.4 mm or 11/16" diameter and not available from many standard sources, also the top hat shims are available in thicknesses greater than 110 thou easily, so if they had renewed the valve seats and not cut them in to the correct depth they may have used top hat shims to get over the issue.

Jag shims are available up to 119 thou, but often to order, the standards are 85 thou to 100 thou. But top hats are easily available up to 220 thou.

Funnily have just been doing the tappet shims today on Wearlej's head today to set his cams up after a head rebuild. And had a conversation about just this today with the guy who does all my machine work.

What have you had to have done ?

 

Car just not running right after having a bucket load of work done, first time I changed the plugs on the newly rebuilt engine two Helicoils came out with the plugs. Decided to remove the head and found a damaged exhaust valve as well which explained a lot. The firm that are currently sorting these issues out wanted to know why the original engine builders used these top hat shims instead of standard ones, wondered it had something to do with the fast road/high lift camshaft setup. When you look online seems the a lot of race guys use these, wondered why. Also how can you adjust them ?
Is it a head off job ?

 

Top hat shims are changed just the same as standard shims, it's just that they sit over the stem of the valve, work in the same way, i'm not sure if the buckets(cam followers) would be changed for a high lift cam setup.

Inside the buckets there are flats that sit on top of the shims, the flats tend to be smaller diameter for the top hat style shim, but that may be the bucket manufacture rather than any mechanical requirement.

The top hat shims tend to be ground to suit, but you can get them pre-ground in different thicknesses just as you can with the biscuit style shims.

You can change the shims with either type with the head on, but you need to remove the cams, then remove the bucket, measure the existing shim and install a new shim either bigger or smaller than the original depending on how many thou you need to open or close the tappet gap.

The Lotus twin cam uses biscuit shims as per the Jag even in race setup , but I took this off a Lotus Elan forum -:
"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.
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