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rocker shaft shims


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I am in the process of rebuilding my TR4a engine , I have replaced the cam with a Kent TT1104n and associated followers and springs and had the cylinder head converted to unleaded,

 

On replacing the rocker shaft today I have found that it is sitting slightly proud and although I have slackened off the adjusters it wont seat.

 

Is it possible that shims are now required to accommodate the higher lift cam ? it is only a road cam and I didn't think this would be necessary.

 

Could there be any other reasons why the rocker shaft wont sit back in place , could it be the valve seating after the unleaded work ??,or the different cam followers.

 

Any advice would be appreciated.

 

thanks

 

Martin

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You might need to shorten the pushrods in order to obtain the correct geometry.

See Kastner's Competition Preparation Manual (which may still be available from TR Register sales website) for details.

Ian Cornish

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I think a stock head is 3.330" thick please check. Facing will have reduced that, but by how much?

 

Kastners comment is, "the Rocker centre line should be at 90 degrees to the valve stem at half cam lift". This ensure the correct wipe across the stem without sending the stem sideways and wearing the valve guide rapidly.

 

Whilst you are at it its a good idea to check your new springs at max lift are not coil bound, Revingtons recommend you should be able to open the valve an extra 3mm at max cam lift (with a lever), to ensure you are not going to get coil binding. They also advise against anything other than standard valve spring rates for a road car.

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On replacing the rocker shaft today I have found that it is sitting slightly proud and although I have slackened off the adjusters it wont seat.

 

 

 

I take it its only a single valve at a time i.e. the one on full lift that is binding, if more than one at a time are binding or any of the exhaust valves then there is something more seriously wrong than a small head skim would cause, which I note you don't say it had anyway.

 

Alan

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Is it possible that shims are now required to accommodate the higher lift cam ? it is only a road cam and I didn't think this would be necessary.

 

 

Martin that is possible and often done but is a very bad solution.

 

The reason is that the rocker "scratches" over the valve tip and

gives it a forward/backward motion.

 

This gives a force that lets the valve and guide wear.

It has to be minimized and that is best done when

the middle of the rocker shaft is precisely on the same

height of the valve tip at half valve lift or a little bit lower.

 

As you told you employ a hotter cam that will have more lift.

To do a perfect job normally even for that case the posts of

the rocker gears must be lowered of an amount that is

half the amount of the additional lift.

 

E.g. if stock is 10mm lift and new cam has 12 you have

2 mm more lift and have to cut the posts 1mm.

 

The wrong length of the pushrods must be corrected.

Normally the TR4 has wonderfull tube type pushrods.

They can be shortened in a lathe.

 

One tube side must be cut at the pressed in heads to remove

the spot welds that connect the tube and little head..

 

The little head can be removed than and the tube can be cut to fit.

It is fully okay only to press the little head in the tube without welding

again but take care that it is fully pressed in.

 

As said above check if that is really necessary.

If only one or two pushrods avoid seating that is normal

if the all are a bit to long you have a problem.

The cam itself does not generate that problem but

what I wrote about cutting the posts is still relevant

but done only very seldom.

Normally people fit the sports cam and wonder about the problems.

Edited by TriumphV8
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