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MG Midget and Sprite Technical - 1275 pistons

I knew there would be more questions re this engine rebuild.
Now bored +.020 but new pistons are not marked to show "Front". Only marking is 020 at 90 degrees to gudgeon pin axis. Does it matter which way round they are fitted to the rod? The are County brand BTW.
Cheers
Bill
W Dunsmore

On mine the oversize # was closest to the carbs

But im not sure it makes a differance or not

Remember the rods are A-B-A-B staggered and maybe #ed on the rod caps

And to stagger the ring gaps on the piston, you DONT want the ring gaps to line up down The side of the piston side

There are 2 differant CR for the AE aftermarket piston

21251 is The low compression at 8.8
21253 is the high compression at 9.75

Prop
1 Paper

I think Peter Burgess said not to worry about the ring gaps, as the rings rotate in use.
Dave O'Neill 2

Typically it does matter as the gudgeon pin is offset to one side and not central in the piston, this is to reduce side thrust loads on the power stroke IIRC.
David Billington

1275 Pistons can be fitted anyway round, there is no thrust side. I think the small bore A series Pistons have a thrust side and have Front stamped on them.

Are you fitting them to the rods yourself? It's one of those jobs that fills me with dread - but then satisfaction and relief when I've done it!
John Payne

I do tend to stagger ring gaps but only from habit as I understand they rotate at around once per 1000 revs so it is not a thing to worry about. I did read(not sure if I mentioned in previous thread) the angle of cross hatching influences the rate of ring rotation. My biggest concern with MGB ring gaps is to make sure they are not lined up with the ex valve cutout. This can allow the rings to open up when the piston is fitted to the bore, the ring can then snap....been there done that :)

Peter
Peter Burgess Tuning

Peter I'm sure you're correct about the rings rotating. I always assumed this is intentional to even out wear, in much the same way that pushrods spin. (There are some nice videos of this happening on YouTube).

Sometimes rings can stop rotating if for example the engine is very dirty or if it has been standing unused a long time.
Just the other day we replaced all the injectors in our V12 CATs and while we were at it we had a peep at the bores with a borescope. Starboard is fine but the Port engine has light vertical scoring in several bores, which we reckon is from the rings sticking a bit initially because that particular engine stood for years on a pallet until it was fitted to replace the previous one which had "stuck a leg out of the bed" (snapped a conrod and poked a big hole in the crankcase). The rings appear okay now, but the witness marks are still there. If it were heavy scoring I'd be concerned because it would probably mean broken rings, but it's barely visible.

Piston-ported two stroke engines typically have pins specifically to prevent the rings turning, so that the ends of the rings don't meet the transfer ports, which will wreck an engine pretty smartly. The downside appears to be accelerated bore and ring wear, in older engine types anyway.
Greybeard

On a 3 part oil control ring do you line all 3 part of the ring parts into one gap or do you offset the gaps of all 3 parts

Prop
1 Paper

David, been reading about offset gudgeon pins. Am I right in thinking the offset would be towards the major thrust side and that this is the left side of the engine from the front on the 1275?
John, Think you are probably right about there being no thrust side - don't think the originals are marked either. Can't check at the moment as the pistons and rods are with the chap who bored the block for him to fit. I chickened out on doing it myself.
Interesting discussion on ring gaps. Last time I built an a series was 45 years ago and I'm sure the received wisdom then was to stagger.
Cheers
Bill
W Dunsmore

for him to fit. I chickened out on doing it myself, I was interested to read that part of the post, how do people fit these pistons to the rods themselves, interesting?
Andy Tilney

I stagger them Prop.
Peter
Peter Burgess Tuning

Thanks peter

Thats what i did also

Prop
1 Paper

Well this all turned out to be a bit more interesting than I thought. Thanks Bill for raising the subject and apologies for the thread drift.

I've never had much to do with A series engines but my Dad was a Mini nut and built several very poky 1275 engines. He used aftermarket pistons (Hepolite) and my memory is that they had marks pointing to the front of the engine.

It led me into reading stuff that I hadn't thought about for years. I knew about offset gudgeon pins, but didn't know they were so common. It seems the offsets are pretty small and probably not noticeable to a casual look. Larger offsets would lead to unacceptable out-of-balance forces it seems.

Then I got distracted by desaxe engines which I assumed were more or less irrelevant as I believed they had died out with air-cooled rotary aero engines, but not so. I was amazed to find that some moderns (Honda, VW) are "desaxe", which is another way to reduce frictional losses at the thrust face without the penalty of piston slap from offset gudgeons. I spent the evening working through the formulae to define the spatial and kinetic relationships that change with different offsets. Not everyone's cup of tea I grant you, but to a maths nerd who loves engines this stuff is solid gold I promise.

Can't remember a miserable wet Sunday evening I've enjoyed more! Sorry about the thread drift - have a good week folks.
Greybeard

Greybeard

Thanks for enlivening my breakfast!

I was very vaguely aware of the existence of engines with offset pistons but never knew they were called 'Desaxed' or any of the detail.

As ever this board informs and entertains!

Best wishes
Mike
M Wood

I remember those Hepolite pistons with the markings! Happy days. I have just purchased some Omega pistons for a 1275 rebuild and they don't have any markings. However the pistons have an offset bowl to improve flow.
Bob Beaumont

This thread was discussed between 29/07/2017 and 31/07/2017

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