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MG Midget and Sprite Technical - New rocker shaft

I Have got a new rocker shaft and rockers ready to fit, but I’m a bit worried about releasing the tension on the head bolts to be able to fit it.
The head is a 12G295 which has been modified by Peter Burgess and torqued down to 50 lb/ft It has a Victor Reinz head gasket which was fitted using Wellseal on Peter’s advice.
So, what do people think, should I risk slackening off the nuts on the rocker side and re tightening afterwards, or do I need to fit another head gasket?
Bernie.
b higginson

It is a risk but I have done it twice without problem on either occasion
Karl Thompson

Why don't you just slacken them all, in the recommended order? Surely the gasket doesn't have to part company with either surface?
Nick and Cherry Scoop

I've done it,just releasing the rocker pedestal ones. Its a pretty rigid iron head so shouldnt lift. After refitting I think I then went round the full set, re- torquing each in turn.

Maybe drain off some coolant to lower it below the top of the block before starting? I dont know!
GuyW

Hi Bernie. Not something done often, so replacing a head gasket at the same time isn't really an issue, and 'normally' you would; -- I would.

In your case, you either don't risk the hg and not use the new rocker shaft, or you do risk the hg.

Nothing to lose really, since you don't really have any options.

You may as well try.

I'd do as Nick suggests, crack the torque on all, so that you can retorque them all evenly after the deed.

And as Guy says, drain down first.

Bonne chance. 🙂. Think of Vera.😁 Or Gracie 😃.

https://youtu.be/_bGE8248N7Y?si=WDvpXV0XPCZIgcfg

https://youtu.be/7EUytEX_XkE?si=8tu_X5F_9F1l8zt1



anamnesis

BTW, what new rockers are you putting in, Bernie?
Nick and Cherry Scoop

Yep, i do as Nick advised--sort of---drop some coolant and undo 'all' the headnuts by 1/4 a of a turn in reverse tightening sequence (start on the ends and work in towards the centre) then another 1/4--- in total half a turn to release the pressure, then undo the rocker gear and take it off---
Then on reassembly retension in steps as normal

willy

Haven't done this on a A series ,but on my mate's MGB racer with it's expensive headgasket---i let all the rocker adjusters right off and removed the shaft lockscrew then pulled/twisted the shaft out leaving the posts etc still tensioned up on the head---It's a bit of stuffing about rotating the engine to get each rocker on the back of the cam putting it back together but if you're really not wanting to disturb the head or gasket, it's an option---
William Revit

I think I've only ever done what Willy just said - remove and replace without disturbing head nuts. Isn't that normally done?
Bill B

I blew a head gasket while on the rolling road at Peter Burgess. I had used the gasket twice (releasing the head bolts and torqing them afterwards.) Peter had a new gasket in stock. I wonder if the rings can be deformed twice.
Flip Brhl

I've been assuming that Bernie is replacing the pedestals, perhaps to get high lift.
Nick and Cherry Scoop

I was going to suggest the same as Willy - i.e. loosen ALL the studs, or back off the rockers and withdraw the shaft.


Willy

The MGB has shims under the two inner pedestals, to 'tension' the rocker shaft. Doesn't that make it difficult to withdraw the shaft?
Dave O'Neill 2

Surely is it not possible to slide the shaft out through the pedestals without recourse to slackening the head?
Alan Anstead

Is it possible

or

Is it not possible

?
Tim C

The stumbling block would appear to be the rocker shaft locator that is retained by a compression plate on top of one of the pedestals. It is cone shaped to locate in the shaft, threaded to fit in the pedestal, with a ‘flat’ that fits a slot in one pedestal plate. The plate is held by a head bolt and pedestal bolt.
Now I expect some to tut-tut but it should be possible to cut the plate and release the locater: it could be replaced with thread lock after ensuring that all traces of swarfe are removed. It really depends on how desperate one is not to disturb the head gasket.
Alan Anstead

If only all A-series engines had been fitted with this retaining plate.

It was fitted to some Minis, at least. The ‘2A’ part number prefix would suggest that it was not a later development.

https://www.minispares.com/2a259-rocker-post-lockplate



Dave O'Neill 2

Dave--as mentioned i haven't tried it on a A series but on a B it can happen,-- just nick through the lockplate up against the headnut then undo the rocker gear nut to remove the plate--or just remove the rocker gear nut and bend the plate up out off the locator stud then undo the locator stud. (sounds rough but when the headgasket costs big dollars you do what you have to do.
With the front and rear rockers removed then poke a philips head screwdriver through the front splitpin hole and twist/pull the shaft out collecting rockers, springs etc as you go---the shims under the 2 centre pedestals make it tightish to start with but as soon as the shaft clears the rear pedestal it's not too bad
William Revit

The head is a 12G295 which has been gas flowed, ported , skimmed and had 1275 valves fitted. All by Peter Burgess.
To line up the rocker arms correctly, spacers have been fitted under the rocker pillars. I guess the spacers would stay in place with the new rockers, which are the sintered ones, standard ratio.
I came into possession of the rockers as part of a batch of midget parts I bought, which had belonged to a now deceased Midget owner, so I’m in two minds as to whether I fit them to my car which has high mileage original pressed steel items, or just sell them on.






b higginson

Bernie, if it was me I think would keep the existing one’s unless there is an issue with them. Will save a lot of hassle, and what would be the benefit?
Les
Les Robinson

The spacers under the rocker pedestals will be to maintain the rocker geometry. Probably equivalent to what has been shaved off your head.
Alan Anstead

I was wondering what the spacers are for, I don’t have any. However I do have a Peter Burgess modified head - a 12G202.
My original 948cc mk2 Sprite had a 12A185, which I am keeping as, to my knowledge, they are rare.
Les Robinson

Les
I used to run a Richard Longman 202 head but don't recall using rocker pillar spacers on that. However the 295 head has a large combustion chamber at 28cc, from memory, so it is necessary, if using it on 948, to remove a lot of metal for those that require high CR thus altering the geometry. I actually fitted mine, using its larger combustion chamber, to lower the CR
Have I got a pot of gold under my bench in the shape of a 12A185 head?



Alan Anstead

It’s not just machining the head that changes the geometry. The camshaft can also have an effect, particularly if it is reprofiled and with higher lift.
Dave O'Neill 2

It is not possible on a 12G295 to shave off the amount equivalent to the thickness of the spacers because of the proximity of an oil gallery to the head surface. The spacers are to make the rocker arm sit square on top of the valve. The cam is a new billet Kent 266, not reprofiled.
b higginson

Bernie

It used to be a common mod on the 12G295, back in the day, to braze up the oilway and re-route it up the stud hole through the head. I had a head like that on my A35. It went like stink, but the deck was a bit thin ;o)
Dave O'Neill 2

There's a bit of misunderstanding here with rocker gear spacers etc.
'Usually'---the bigger the valve lift, the 'lower' the rocker gear needs to be.
For proper operation/geometry there should be (at right angles to the valve stem ) At 50% valve lift of the cam being used,---- a straight line from the top of the stem, through the centre of the rocker shaft and through the centre of the ball on the adjuster screw, then a pushrod (usually shorter for higher lift cams) can be selected to suit.
Just poking rocker spacers instead of measuring it up properly is a bit of an easy way out for a head that's been shaved, but not-----ideal.
If using roller tipped rockers , then sometimes spacers are needed as the line then has to be through the centre of the roller- not the top of the stem.

willy
William Revit

Bernie
I missed seeing the pic you put up of your rockers=

As it's a full assembly, yep , I'd just let the tension off the headnuts as described earlier and then just remove the old assembly complete and fit your new set and retension the head up as normal

willy
William Revit

This thread was discussed between 23/08/2026 and 29/08/2026

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