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MG MGB Technical - Replacing tappets

Hi all,

I am replacing the cracked head in my MK2 with a "warm" aluminium one.
The ally head is lower by approx 4mm over the standard.

Hopefully the adjusters will have enough movement to take up the change,
If not can I shim the rocker posts?
or
Replace the tappets and pushrods with the 18V type WITHOUT replacing the camshaft?

Terry
T J Malloch

I don't see why not as long as the rockers will still clear the cover, and oil holes are provided for. The middle two posts should be shimmed anyway to prevent the rocker shaft twisting in the posts, make sure you have additional shims there again.
PaulH Solihull

The later bucket type tappets & longer pushrods are the same length as the earlier type set up but the tappets are much lighter. It is not advisable to pack ther pedastals much higher as it affects the angle of the rocker onto the valve end & will waer the valve guides more. Many worked heads are 2 - 3 mm shorter than standard so you will probably have enough adjustment.
Garth
Garth Bagnall

Otof interest, is this a cross flow head, or standard, just in aluminium? Although it is lower, is the estimated compression ratio the same? Mike
J.M. Doust

Hi,
Garth,Paul,
I have just done a trial fit and can't get rocker clearance, I need to fit shims or source shorter pushrods. Another option(which I don't want to explore at present) is a reground cam with a smaller base circle.
Mike,
When I bought the head I was told it had a C/R of 9.4, which approximately what I believe I was running after shaving .040in off. I will have to check this.

Summer is coming and I want my toy back on the road and faster of course!

Terry
T J Malloch

Ideally you need pushrods shorter by the same amount as the head is shallower. However if these are not available fit shims under all the rocker posts to the same thickness as the head is shallower. The rocker geometry will not be totally accurate but it will be near enough and get you running. Don"t forget the rocker shaft oil feed hole in the front post. Valve guide wear should not be too heavy.

Jim
jim soutar

Can one get shorter pushrods? is it possible to shorten the existing ones at the non-cup end of course, but then one needs to re-harden? Can an engineering shop do this or could they source shorter rods> Advanced Performance Tecnologies might be able to help, from the USA. Mr. Strange you know these guys? Mike
J.M. Doust

If you decide to use shorter pushrods cut and shut them instead of trying to rework the ends. It would not be a difficult task for a competent welder. Before doing anything, trial assemble the rocker shaft onto the head & see what action you get with the rockers onto the valve. Use a screw driver to open & close the valve. Then put some washers about 4mm thick under the pedastals and have another look.
Check also that the alloy head does not have the offset rear pedastal oil feed hole as on late heads.
Garth
Garth Bagnall

The 18v pushrod/tappet combo is 3 mm shorter than the pre 18v short rod/milk churn tappet combo.

Peter
P Burgess

I was wondering why I had lost nearly a 1/4" of adjustment when switching over to 18V tappets and pushrods after an engine rebuild. This explains a lot. Thanks, Peter. RAY
rjm RAY

It helps with skimmed head applications though Ray.

Peter
P Burgess

Peter, my cylinder head was skimmed when I overhauled the engine. I did install a Kent 285 camshaft at the time. This may have been a reground unit and that would explain the loss of adjustment threads. RAY
rjm RAY

I had the same problem Smith Bros Pushrods did an excellent job of custom length pushrods He has worked with APTfast and has the correct ends I assume he can ship downunder http://www.pushrods.net/
Charles O'Brien

Hi Ray

A skimmed head would require shorter tappet/rod length, a repro cam would need a longer tappet/rod length. The preoblem is worse with a billet cam.

Peter
P Burgess

This thread was discussed between 15/10/2011 and 21/10/2011

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