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MG Midget and Sprite Technical - 1.5 rockers?

My 1098+40 thou Sprite engine is currently running a Kent 266 cam with 4 degrees advance as advised by Peter Burgess. It also has an HIF44 carb. It has the standard pressed steel rockers. The head is not ported or gas flowed, so would fitting 1.5 rockers give me any more performance or would I be wasting my money?
Bernie.
b higginson

Bernie,

Simplex or duplex chain? I had a 276 in a 1098 and it was fun while it lasted, especially when later fitted with a 12G940 head, but I had been warned and the simplex chain didn't last. I expect higher ratio rockers wouldn't be a good move unless a duplex chain fitted.
David Billington

It ought to give some improvement.

When I tried some 1.5 rockers on a 1275 with MG Metro cam, it felt much livelier over the standard rockers with the same cam.


David,

What did you do about the compression ratio with the 940 head? IIRC the combustion chamber is about 25% smaller than the 12G295.
Dave O'Neill 2

Dave,

It's getting on for 40 years since I did it but IIRC the CR came out OK when taking into account the larger gasket volume and the pocket in the block.
David Billington

I run 1.5 roller rockers on my Sebring Rep. They do sound like a sewing machine that might not be everyones choice of music.
They are specced to the other engine mods by MED. Engine builders such as MED spec there engines based on science ie dyno’s whereas trying to spec an engine by guesswork can lead you on a possible expensive wrong route and also waste a lot of time.

Alan Anstead

Always worth reading David Vizard’s book, as he did a lot of testing and development.

Saves guessing.
Dave O'Neill 2

Dave B
Fitting a duplex chain is about maintaining cam timing reliability.
With the simplex as it is more prone to stretching over time the camshaft timing with alter.
Alan Anstead

Alan,

When the power in that engine dropped off I replaced it and stripped it and I found little half moon pieces in the sump which I soon realised were the rollers from the chain having broken up, not all, but a significant amount. I had been warned about running a high lift cam with a simplex chain and that was a result. It got replaced with a 1215, 1098 block Imp pistons, a very nice engine IMO.

Regarding the 12G940 on a 1098 Barum Engines did one last year as shown on their Youtube channel, they pocketed the block but they had an issue which seemed to be the head nuts bottoming out on the stud thread from the symptoms but I never saw any more information on a resolution to the problem. A comment was made by someone that had worked for BMC Australia IIRC and when the 12G940 head came out they had access to the engine drawings and worked out it should just drop on which the commentor did with pockets, a colleague didn't pocket the block and all was fine but when the head was later removed the top of the block below the exhaust valve was clean the clearance was that small. That was a stock cam AFAIK, higher lift would have caused issues.
David Billington

Yep, checking clearance at full lift is essential, specially with the higher ratio rockers.
David, I've been there with the disintegrating chain issue, bit of an eyefull when you see all the little half rollers eh.
When I get chains now i make sure they have full circle rollers ,not the split roller type.
William Revit

My engine has a Duplex chain (second one, first cheaper one had stretched)
12G295 head, light skim. Maniflow full exhaust.
b higginson

Give it a go, it will just lift everything a few hundred rpm upwards. don't forget to increase valve clearance by % increase in ratio
Peter Burgess Tuning

I tried some 1.5 ratio roller rockers on my car for a while, but took them off again. I didn't like them, but then I don't use the car in competition. I found them very noisy and, as Peter says, they moved everything up the rev range. For my driving, often on hilly or twisty roads in the grim up north country, I prefer low down torque to a screamer engine. Not that mine was exactly a screamer but most of my driving is at below 5000rpm and the 1.5 rockers didn't really help there.

I sold the purple roller rocker kit back to Nick Veuger, for those who remember him, exchanging them for the 1098 engine now residing in my Frogeye.
GuyW

I run my cars uphill & down dale at home and all over Europe. Sometimes I will arrange a tour for people (the pictures are from a Swiss Alpine tour that I arranged) but I am equally happy driving in Europe alone.

I have MED 1.5 roller rockers, MED HT camshaft, MED cylinder head and twin 1.5 S.U.s
I have been very pleased with this set-up over 20yrs.




Alan Anstead

This thread was discussed between 27/02/2025 and 03/03/2025

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