MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Pistons without offset?

Dear All,
Bit of paranoia. Got my local machine shop to fit pistons to the rods of my 1275.
Strange thing is that the pistons (County brand) have no arrow denoting the front of the piston. The only mark they have is the overbore size.
I have checked for gudgeon pin offset and can't see anything. Have read some modern pistons have no offset (have even seen mentions of 1275 pistons not having offset)
Is this true? If yes, then I guess there is no preferred orientation?
Interestingly the machine shop seems to have marked one boss on each piston with a green marker and when fitted these are all at the front. Although this doesn't leave the overbore markings all on the same side?
Thanks for any help
Cheers
tim
T Dafforn

Pins are not offset.
The dish sometimes is.
Onno K

The offset is on the rods onto the crank. That will be why the paint marks are there. But it is also good practice to avoid having the compression ring gaps on the thrust side of the bore.
Guy Weller

great tip guy...was that in the hand book...haha

you dont want the gaps to line up on top of each other as well

prop
Prop and the Blackhole Midget

County piston manufacturers tend to use blue red or green dabs of paint in selecting piston sets. I cannot measure a difference in sizes so assume they deliniate matched sets for weight as part of the QC. I was not aware the 1275 std piston had any offset as the B series std piston does. I tend to use Mini Spares Mega pistons in our A series engines as they are good value and high spec. You can always check for offset by pushing the gugeon pin into one piston but leave enough to push a second piston on the pin, you should see a visible difference if they are offset.

Peter

Peter Burgess Tuning

None of the 1275 pistons I've fitted have been offset. I always fit them so the oversize markings all line up when in the block though. It avoids confusion later!

Its one job I hate doing, I like the satisfaction of having done them but it's a worrying time! I messed one up once and ended up with a pin stuck about 1/4 inch out. There wasn't much else to do but put it in some 'V' blocks on the press and force the pin to the right place. Still going strong.

John Payne

Had the same thing.
Then found out my machine shop has a special tool to fix those FU's

Just dropped of my cam there so they could check before fitting the cam bearings.
Then I'll be getting it all back soon and it will be time to put the pins away cold and heat up the rods again.

Now hope I don't need the FU tool again
Onno K

Thanks chaps. Looks like no gudgeon pin offset.
Had mine fitted by my machine shop. The seemed pretty knowledgable.
I'm guessing that they had seen a fair more A serieses than the lotus twincam the did for me last and that really whizzed when rebuilt.
Cheers
Tim
T Dafforn

...

Greg H

Hi Greg,
Thanks for the clear image.
I was really talking about the offset that the often is on other engines on the gudgeon pin boss to reduce piston slap.
Looks though that for some 1275 pistons there is no offset.
Pistons are now all installed and everything is moving smoothly.
Cheers
Tim
T Dafforn

Hi Tim yes realised what you were talking about but thought the pic showed how the offset was in the rods only and the rod was meant to be central in a symetrical piston.
Greg H

Ok. See what you mean. but I think the piston boss offset is 90 degrees to the Conrod offset. Or maybe I'm just mire confused!
Cheers
Tim
T Dafforn

I'm thinking there is some confusion here! Tim is (I think!) talking about the pin being offset in the piston - I think it is what is sometimes called the thrust side. Thats why some pistons have an arrow so you fit them pointing to the front of the engine. Like Tim says, at 90 degrees to the crank offset. Nothing to do with the centreline of the pin being different to the centreline of the big end - which is why the rods are offset as in the diagram above.

I wish I had a diagram to show it instead of my confusing explanation!

Or maybe I'm confused - anyway I think Tim has his answer!!
John Payne

John. You got it exactly.
In my case pistons have no arrow and no offset, but conrods do have offset.
Cheers
Tim
T Dafforn

I mentioned the offset crankpins in relation to a possible explanation for the green paint marks, - not as a comment on the question about offset pistons which is quite a different thing. But as the pistons and the rods are already assembled this would influence which way around the pistons were fitted in the bores. And hence my further comment about the positioning of ring gaps, as some pistons will have pegs in the grooves which determines where the ring gaps end up.
Guy Weller

This thread was discussed between 10/03/2013 and 12/03/2013

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now