33 replies to this topic

[Bradley Gaunt]

I have recently purchased a 1380 engine and on inspection I have noticed that the distance between the pistons is between 3.3 to 3.6mm (3.3mm between 2 & 3). Do you think this has been offset bored and if i were to get a new block in the future and had it offset bored even more would the crank need to be modified or is the difference made up with movement between the piston and con rod? I have also noticed a tiny crack between the 2 middle pistons also and am having it inspected next week with the intention to having the block skimmed and hopefully the small crack machined out.

[bmcecosse]

Unlikely you can 'skim' a crack out! The offset boring relies on the slop in the con-rod/piston alignment. Better to just stick with standard bores - the cracking is typical of the problems............

[Bradley Gaunt]

Hmm, well I guess I'll find out next week. Do you know what the normal gap between the pistons should be on a 1380, if it wasnt offset bored what would the gap be?

[Cooperman]

At 1380 if you have any sort of crack between the walls of 2 & 3 then it is likely that you need a new block. If the 'meat' between the bore edges is only 3.3 mm then it sounds like it has not been offset bored. You can't really sleeve a 1380 even if it has been offset bored.
That's why a lot of people stick to 1330 (i.e. +0.060") as when a 1330 bore needs re-boring you can sleeve back to 1275 and save another block from the skip.
The questions are where did you buy it from and can you get your money back?

[MRA]

If you offset bore more than the prescribed amount then you will be .....

A) Scrapping the block
B) erm .... scrapping the block

If yo ugo too far you will cut in to the oil way which in some instances can be sleeved however best to avoid than to risk it.... why would you want to offset more than the normal amount ? there are 1380 turbos putting out 300bhp without any issues....

[Cooperman]

The OP is concerned that it has not been bored with an offset at all and also that there is a crack between 2 & 3.
I believe the correct offset is 1 & 2 by 0.015" towards the front of the block and 3 & 4 by 0.015" towards the rear of the block.
If the machining is done accurately, then a 1380 will work well, although there is no guarantee that the block can ever be bored further and at that size it can't be sleeved back, or so I'm given to understand, although I've never tried it. Some blocks can go further, but it's a matter of luck.
I guess it's been done by another 'cowboy' machining company who are not A-series specialists. One 1380 was brought to me where the block had not been offset and the bores were oversize, one by 0.004". The engine was built and run which caused 'ripple' effect up 2 of the bores and a broken ring. Result - another scrapped block!

[MRA]

I had a customer bring me a similar block but it had been offset the wrong way.... the company concerned said it wasn't their fault as its a mini engine and they should all be scrapped anyway......

Obviously I cannot name & shame them for legal reasons

[Cooperman]

I've never understood this desire to go to 1380 on a road Mini or MG Midget/AH Sprite. For competition the advantage of 1380 over, say, a 1330 is obvious as the extra 4 to 5 bhp will represent a faster lap time or a quicker time up a hill-climb course and that's good. But for a road car the difference in performance will probably not be noticed and considering the potential cost if it all goes wrong it seems hardly worth it. After all, a worn out 1330 can be sleeved back to 1275.
To do a 1380 properly a trustworthy machine shop is vital and one which fully understands the A-series engine and knows what they are doing. Such companies seem to be increasingly rare these days - except those on here of course! Now, a properly done 1380 is a very driveable engine, but it must be done properly.

[Bradley Gaunt]

I've been on the phone to mini sport and they can supply me an exchange block for around £165, ill replace the bearings at the same time. Hopefully this way I'll get what I need and it will not pop head gaskets all the time! Still happy I made the purchase as the rest of the engine seems good - titan 1.5 full roller rockers, stage 4 head with 38mm and 31mm valves nitrided cross drilled crank with omega pistons and cooper s rods. Also the guy I have bought it off has gone somewhere to meeting me half way on the replacement bits.

[Cooperman]

Are you sure they are Cooper 'S' rods? The 'S' had an EN40B forged steel nitrided crank with smaller big-end journals, so an 'S' rod won't fit a non-'S' crank. If you have a genuine 'S' steel crank with nominal 1.625" dia big end journals instead of 1.75" big-end journals as found on all other 1275 engines, then you really do have something very special and very strong.

[Bradley Gaunt]

Thanks Dilligaf, i will check, in fact I asked the guy to ring me back with details of the gaps between the bores to make certain they were better than what I had... still waiting for the call lol. I know MED do a good job but can anyone suggest somewhere that does a good offset bored block?

[MRA]

How are they going to "size" the bores to your pistons ?

[Cooperman]

I don't think you will be able to exchange a cracked block.
Best bet is to buy an un-re-bored 1275 block and have it bored to suit the pistons by a reputable engine builder, instructing him to offset bores 1 & 2 0.015" towards the front and bores 3 & 4 0.015" towards the back. Then you will know it will be fine.
Alternatively get a 1275 block and a set of +0.060" pistons and have it bored without offset to +0.060" and go to 1330 cc.
Martin at MRA will, I'm sure, be pleased to help with this.

Edited by Cooperman, 20 August 2011 - 08:25 PM.

[MRA]

Really, I didn't think Steve had any boring equipment, he subs it all out

[Cooperman]

You need to get a new block, ideally at standard bore. Then get it bored to suit your pistons if you want to stay with the ones you've got. MRA could do this I'm sure. Then do a 'trial-build' and measure/check everything including measuring how far down the bores the piston crowns are at TDC. Strip it down again and get the top of the block 'decked' to bring the piston tops to within about 0.003" of the final deck height. Use 'Plastigauge' to check the running clearances of the bearing shells, mains & big-ends, to the crank surfaces.
Clean everything thoroughly and finally assemble.
Did you check to see if it's an 'S' crank & rods?