Shims

[saildrive2001]

What is the recommended range of shim sizes for the Twink? I have just had new valves & seats installed & I think my gaps are too large requiring too thick of a shim.

[rgh0]

If you need a shim over about 3.5mm or 140 thou when running a standard head and standard cams and cast iron cam followers then you probably have something else wrong that should be fixed. If using non standard components then a larger shim requirement may be an outcome, conceptually there is no really upper limit to thickness of shim you could use if the set of components you selected required it.

cheers
Rohan

cheers
Rohan

[saildrive2001]

I have a UK spec Stromberg engine to SE spec with all standard spec for that model. I have some gaps up to 160 thou so I am thinking that the new seats were not cut deep enough.

[rgh0]

You either have the valves not set deep enough or some other component is shorter

eg valve stems not correct length ( were new valves fitted?), cam base circle smaller ( has it been reground?) cam follower buckets thinner ( were new thin steel buckets fitted?)

Some of these can cause other problems

eg If valves not set deep enough or valve stems to short then the installed spring loads will be outside the correct load range
.
Other may not be a problem and can be safely accommodated by thicker shims

eg thin steel cam followers or smaller cam base circle.

Your head machine shop should know what dimensions they set the valve heights to and have set the distance from the tip of the valve stem to the cam centre line at the correct distance if they truely know what they are doing with a twin cam head.

cheers
Rohan

[saildrive2001]

Hi Rohan,
The cams are original, checked but not needing grinding, buckets, stell spring seats caps & collets are original, new valve guides. New valves from Dave Bean are standard valves & are as per the old ones. I didn't have any info on the height of the valve stem to the centreline of the camshaft. This m/c shop do lots of alloy heads but may only have done one twink before. They are careful but I think they didn't machine the seats deep enough. Do you have a dimension from the valve tip to the centre of the camshaft?
regards,
Keith

[rgh0]

Cam centre line to tip of valve = 23.37mm / 0.920 inch
Tip of valve to bottom of spring pocket = 29.97 mm / 1.180 inch
Bottom of spring pocket to cam centre line = 53.34 mm / 2.100 inch

The Dave Bean catalogue in the engine section has a diagram that gives most of these dimensions also

This should give you a shim thickness with all other standard valve train components of
Inlet with 6 thou clearance shim = 2.95 mm / 0.116 inch
Exhaust with 10 thou clearance shim = 2.84 mm / 0.112 inch

160 thou shims implies the valves need to be about 0.050 inch further into the head.

regards
Rohan

[saildrive2001]

Thanks Rohan,
I have the Bean catalogue but I forgot to look there. I agreee the seats need to be cut deeper.
Thanks again for your input.
regards,
Keith