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MG Midget and Sprite Technical - High lift rockers

The glacial slow engine rebuild for my Sprite is nearing completion. However there is one matter holding things up - the rocker gear.

It has been bored to 998cc (approx +66 thou, to take standard size Mini pistons) & has a Swiftune SW5 cam. My brother who is doing the work tells me that the standard rocker gear is nowhere near able to handle the higher lift cam, so I'm needing roller rockers. However 998 roller rockers seem to be a huge amount more expensive than 1275 ones, which is annoying.... Is there a way around this, e.g. using a 998 rocker shaft with 1275 roller rockers, or are the parts completely incompatible with one another?

Also, there seem to be various suppliers - is there actually much (or any) difference in the actual product?

I gather from bro that the Vizard book is mentioning shorter pushrods, which he reckons is worth looking into due to the cam lift. Are these available off the shelf, or is it a case of shortening the old ones?

Lastly, does anyone have a set of the correct factory air cleaners? IIRC, these are the type that have radiused corners, rather than the chamfer of the later ones. The engine previously had open trumpets, and I don't want inflicting those on a fully rebuilt engine, or onto the soon to be fully rebuilt carbs!
Paul H

What ever rockers you use, Check for coil binding before starting!
Also check minispares for info on rollers, high lift or normal.
Alex G Matla

If you need shorter pushrods can only be determind by a trial engine build!
But i've always managed without them.

The standard rockers wil be able to stand the lift of an sw5 with ease.
But you do need 998 rockers if you want to go roler.
There is diffrence some wear more than others.
Keep an eye out for the diffrence between roler tiped and full roler.
Nothing wrong with roler tiped only if you pay ful roler price for them.

You can use the trumpets if you just put a filtersock over them.

What carbs will you be running?
Onno Könemann

Cheers for the info regarding 998 vs 1275 rockers. I thought that would be the case, but wanted to double check just in case!

"If you need shorter pushrods can only be determind by a trial engine build!
But i've always managed without them.

The standard rockers wil be able to stand the lift of an sw5 with ease."

The engine is fully built up, aside from the rockers. There has also been a load skimmed off the head after the valves were de-shrouded, to correct the compression ratio, which may be affecting the pushrod length too.

Ideally, my wallet would certainly prefer regular rockers, & I will check about the pushrod length, but from what I remember him saying, he thought it would need shorter pushrods as well as the roller rockers, due to the angles involved. I will check though. The standard rockers that came with the engine are shot anyway....

Regarding the trumpets, I won't be reusing them, as I want it to look correct (the car is a very early rhd Mk2), hence also tuning the 948 rather than going 1275.
Paul H

Then get the 998 rockers fit them and use the corresponding chapter in vizard to determin wether the you need shims under the rocker posts and/or shorter pushrods.

You will be using K&N replacement filters i hope?
And you can drill some holes in the botom of the original filter case.
They can not be seen when fitted but make it a bit easyer to breath for the engine since the original pipe can be restrictive.

Nice to hear you want to keep it looking original, anny pictures of the rebuild?
Onno Könemann

Paul,

My Frog is bored to 998cc, hi-compression mini pistons, 12G-206 head -50thou, Swiftune cam and springs. I'm using standard rocker gear and pushrods.
No problems.

Dennis
D Stanfield

This thread was discussed between 09/09/2010 and 12/09/2010

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.