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MG MGA - oversize bore

How far can you bore out 1588 engine? 60thou? Bob

I suppose in theory 1950cc is achievable, as it is the same as the MGB 3 bearing block. I undertsand though that at the larger bores (1900cc+), thee can be issues with the crank as the 3-bearing isn't as strong/well supported as the 5 bearing crank in later B series engines.

Don't quote me, but I suppose there would be no problem taking it out to standard MGB bore, maybe +40 on that.

All the best,

Grant :-)
G Hudson

I suppose there is no chance to ever bore a 1500 out to 1800cc without breaking a cylinder wall. 1500 and 1800 have different block castings. 1800 starts with larger bore, approximately same cylinder wall thickness, smaller water jacket. 1800 has been successfully enlarged to 1950 fairly often (not always successful). Trying to push the 1800 block to 2000cc will I think more often fail than succeed.

Therefore, 150cc oversize may be a reasonable goal if you can measure the cylinder wall thickness first. It will depend on how well the sand core was aligned for original casting. A slight shift of the sand core will make the cylinder wall thick on one side of the bore and thin on the other side.

All said, there is at least one example of a 1500 block being bored out to 1698cc. See here:
This must be about as tricky as taking the 1800 out to a full 2000cc.

My philosophy is to take the minimum rebore to get it to clean up, and save the rest of the meat for the next time it needs more work. If you bore it to the max it cannot be repaired again without sleeving (expensive). If you bore it that far oversize, then boring for sleeving may likely break into the water jacket, in which case the sleeve would have to be soldered in place, not easy to do with a large casting.

There are quite a number of Twin Cam engines that were enlarged from 1600 to 1762cc using a factory supplied kit. See here:
I have never hear of one of those failing, so it may be reasonable to punch out a 1500 by a similar amount with some confidence. But it is possible that the Twin Cam block may have slightly thicker cylinder walls.

In any case where you intend to bore a block more the 0.060" oversize, you should always measure the cylinder wall thickness before you start boring. If a finished cylinder wall is too thin, even if it doesn't break it may warp after thermal cycling in normal use, resulting in high friction, hot running, high oil consumption. I guess if you have plenty of time and money, and an extra 50cc means that much to you, then it's your risk to take.
Barney Gaylord

The 1489 cc engine had water jackets that completely surrounded the bores. The 1588 cc and later all had siamesed bores so that they had no water passage between 1 and 2 or between 3 and 4. The 1489 is least able to take a big bore. The 1588 can be taken out to about 150 cc over size, as can the 1622 and 1798.

I've done a number of 1950 cc engines starting with early 18GB blocks with no problem. Barney is dead right about it getting iffy past that - I bored one out to 2000 just to see what wall thickness would be. Turned out that it had inclusions in the casting that made it pinhole when I honed it. If the inclusion had been a little less thin at that point, and I didn't sonic test it for wall thickness, I could conceivably have built up an engine that would fail in service.

Your 1588 will easily take .080" or more. If you can't find pistons for it, see if you can find pistons for the only slightly larger bore 1622. That will give you a flat crown that with the smaller combustion chamber found on the 1489 and 1588 will give you a nice bump in compression. Note that the wrist pins on the 1622 are larger. You'd have to hone the original rod, or better yet source some 1622 or 3 main MGB rods (NOT later 5 main rods as they won't fit).
Bill Spohn

If anyone is looking for a 1600 engine, I expect to receive one in part-ex for a 1622 in the next few weeks. It will need a rebuild as it has been run without enough oil.
dominic clancy

The company who bored my 1489cc engine up to 1588 said you need to keep at least 0.120" cylinder wall to safely get away with a new bore. We discussed doing a 1588 + 0.040" but I was warned it probably would result in too thin a cylinder wall.

I guess the same dimension would apply to any B series engine.

P Reardon

I'm running my 1600 at 60 over with no problems Bob.
gary starr

I also have .060" over on my 1600. The manual gives a recommended maximum of + .040 but most parts suppliers list .060" rings...........Mike
m.j. moore

Thanks for the help/information much appreciated Bob

Bob, at first I couldnt bring myself to read this thread in case it was being directed at me!

I must get a little carried away on the subject of MGAs as my friends have occasionally used the title of this thread to describe me!
"Oversize Bore" Didnt even think I was overweight!!


Colyn Firth

Gday Colyn, I read your posts on this BBS with great interest.Oversize!! do you fit in your MGA easily? Cheers Bob

When I bought my 1588cc MGA1600 it had already been bored oversize and had 1622cc pistons with the necessary con-rods. After I rebored it, it is now +0.060 up on 1622cc with flat top pistons and a 1967 MGB head. Goes very nicely at 1685cc on 99 octane..!
P N Tipping

I thought you could bore a (late) 1622 block to 1800, and whatever oversize 3 bearing pistons are available. That is only the 1622 block that uses the B water pump though.

Colin Parkinson

This thread was discussed between 06/06/2011 and 09/06/2011

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