Overbore...

[Tim-D]

It s been a nice sunny afternoon so though I'd spend an hour or two on all things mini - some time ago was given as part of a garage clearance a 1275 engine - no one wanted it as the box was shot but owner swore it was very quick - and had retained it for a future project when its home rotted around it - normally I treat such things with a large pinch of salt - the really grimy engine had therefore sat outside my garage headless & under a sheet of plastic for the last year - today was the first time I'd given it a second glance - however a) the engine number 12HC14AA115049 - (notionally came out of a metro HLE) was not affixed to the block with the normal dome rivets but instead had been put on with filed down ally rivets b) pistons once decoked a little are marked 21251 +030, - 2+2 implies has had a + 30 overbore - what does that take capacity to???? 1310?????, cam is unknown does not appear marked at all, gearbox was a goldseal with a 3.65 FD and reason for faliure very evident & an easy fix. Now reason for question other than capacity - block was painted black but underneath it's red and I had thought this colour was only on MG metro's - is it conceivable that I've got a bored out MG with an unrelated engine plate perhaps from the original donor transplanted??? If so is this legal as appears a little dodgy to me!

[love machine]

OE pistons aren't anything to get excited about up to +60. Just quote 1275 and that is fine. Of course the engine has been reconditioned! I think most A+ engines were red. As you say, an HC engine isn't anything to get that excited about. Kent mark their cams on the sprocket end and I think piper do also. The rest are down to markings on the shaft. I'd have a look at the head and use that as an indicator to whether or not it's a performance engine. If the engine has been decked, they may have put the plate back on due to fear of the stazi.

[guru_1071]

that engine has been rebuilt with the horrid cheap low compression pistons. a HC engine would have had 21253 pistons fitted (though with rovers own part number).

the engine plate will have been removed when the block was bored and decked (if done correctly)

all big bore factory a+ engines where painted red (or black if a factory recon (not in this case as it would have had a factory engine number)

it sounds like a cheap 'get ya going' motor or usefull to rebuild - still, if it was free...............

[Tim-D]

mmm - i think I'll take off the ID plate & see what if, anything lies beneath, fitting a 3.65 diff would seem to suggest someone at some point was aiming at a budget screamer - fitting a goldseal box with, did I mention a SC gearset, does suggest some money being spent probably some time ago as +30 overbores seem a rare thing. Am I right in thinking that the downer on the 21251 piston is down to they're being "normal" compression that could potentially be corrected by decking the block? - or would this be a generally unsatisfactory mod? - still reckon this was a budget effort as centre strapping & centre pick up pipe conspicuous by their absence - cam appears completely unmarked but will take a much closer look! - yep it was a freebie so just may be worth awaiting some tastier components to come along as bores are A+ and it's no hard store......... :-)

[FWDRacer]

Whilst the pistons aren't ideal, as 21251 Pistons don't live very long if the engine is repeatedly revved over 6K, if you build an engine that is torquey and delivers its power at 5.5-6k rpm they'll be fine.

Built a budget 1330cc Motor with these pots (+60) and it gave well over 85bhp and importantly 92lbft at 3250rpm. These figures, whilst not making any A-series headlines make a Mini very quick as the performance is accessible in everyday motoring.

[Cooperman]

I normally use either 21253 or, in my 'S' unit, Karl Schmidt pistons (c. £450 per set, if you can find any). However, I have used 21251 pistons in a 1293 engine and decked the block to get the right compression ratio. Power was 94 bhp at 5750 with a 276 cam. They are fine so long as over about 6000 rpm is not used in a sustained manner and for a fast road engine they are probably much better than the old 60's crap with which many A-series engines were originally delivered. The weaker crown of the 21253 is not so much 'weak', as not quite so strong as the 21253 is. Modern alloys are so much better now and manufacturing tolerances much tighter as well.

[guru_1071]

[quote=Cooperman] The weaker crown of the 21253 is not so much 'weak', as not quite so strong as the 21253 is. quote]

pete, number fumble, i guess you mean 'crown of 21251?'

the other problems that the 21251 have (over the 21253) is that the ring land area is weaker (hence why they are crap for high revving or high comp) and the rings are a cheaper design and just dont take the hammer that a 21253 will, which to be fair is a very good piston for the price

[Cooperman]

Yes, Rich, I did mean 21251.
I always use the 21253 in engines I build for other people where bhp is important above all else. My point was that for a slightly improved road engine the 21251 is not a bad piston; it's just not as good as the 21253.
For a 1275 'slogger' unit with, maybe a 266 or even 276 cam, 10.3:1 c.r and fairly high gearing the 21251 would be a low-cost option which would work for quite a long time. For a high-revving (say 7000+) 110 bhp+ unit using a 286 or hotter cam, then the 21253 is fine but definately not the 21251.
I built an engine for an Historic rally Mk1 Sprite recently. It was 1115cc and the only pistons I could get were the old 4-ring ones. I was nervous about using them, but I balanced them, checked them and made the c.r. only 10.2:1. With a 276 cam, twin 1.5" SU's and a 'nice' 295 head it gave 74 bhp at 5600 which was fine considering the low weight and low gearing. I mandated a max of 6000 to the owner and he has been carefull not to exceed this (or so he tells me!).
I've just built a 1063 cc (998 + 0.080&quot unit with a big-valve 295 head, single 1.5 SU, Cooper 3-branch, etc. I used the Mini-Spares flat-top pistons (Hastings?) which look very strong with excellent ring design. It should run for the first time the week after next. Then I'll need to fiddle about with the needle to get the mixture right as I think it will be too weak at mid-range with the standard 998 Mayfair one.