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MG MG Y Type - Big Pistons?

When I had to remove the cylinder head of my YB for the first time since I got the car, I found the pistons were already last oversize, and again rather sloppy in their bores. Now I know where my oil is going apart from the garage floor! Complete engine rebuild to plan (and save up for) in the near future, I'm afraid. Now I know that established procedure would be relining the cylinders and fit standard-sized pistons. One engine expert (albeit rather with Austin-Healey engines) told me this was not the best idea, because heat transfer from the liners to the block would never be as efficient as before. And there would be an additional danger of the liners working loose. He suggested having forged pistons made to the existing bores, which would only be skimmed out to make them cylindrical again. Advantages would be a much better piston quality, the possibility of using slightly domed piston crowns to raise compression, and a "free" capacity increase. Has any one here made his experiences with either approach to this problem?
Remo Peter

I've spoken with mechanics about the same thing in the past and received the same answer as you. Heat transfer and sleeves breaking loose are both real concerns. If it were me I would simply bore to the next possible size. The engine will not look any different and probably not sound any different either but you may get a few more horsepower which wouldn't be a bad idea for a Y as we all know!
Steve Simmons

What do you call the last oversize? Some original sources did not recommend going as large as 60 thou. over but it is quite common nowadays to go out to this oversize. Pistons are available to go up to 100 thou, and more I believe, and some people have gone further (1350 cc is not uncommon) depending on block thickness.
ian thomson

It is +60 now. Largest according to original workshop manual, if I remember correctly (the manual was recently stolen from my back seat where I had forgotten it - now I wonder who will enjoy reading it so much he wants to have a Y himself!).
Remo Peter

Remo - I am running with +100 on my TD and have had no problems. Some people have even gone as far as +120. beond that things get rather iffy and some wind up in the water passages. Moss Motors has +80 pistons and +100 are obtainable without resorting to special made pistons. Considering the number of cars running around with sleeved cylinders, I really have to question the extent of the problem with heat transfer and sleeves working loose. I think that a compentent machine shop should be able to fit sleeves (when the time comes) without any detrimintal effects. Good luck - Dave
David DuBois


I've been running resleeved XPAG engines for the past thirty years without any trouble - I even did some T-racing at the time.
Anton Piller

This thread was discussed between 04/12/2003 and 09/12/2003

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