12 replies to this topic

[Oggyp]

Hi All,
I bought a unfinished project about a year ago and amongst some new parts that were included were some new 73.5mm pistons.
I have a 1275 A+ engine that would need a rebore anyway so I think its the right time to overbore it to use these pistons to create 1380cc.
I have a very good friend who has a machine shop who can skim heads/blocks , carry out boring and honing etc who will be doing the machine work for me. He builds race spec cortina engines for his historic cortina touring cars along side day to day engine rebuilds so is well set up for the job.

Due to conflicting advise between local mini fans my question is;
Do the bores 100% need to be offset bored? And more importantly if so by how much and in what respect?

One guy I spoke to recently who runs his own 1380 mini in sprints/hillclimbs says offset boring is only neccersary on 74mm+ bores and
he runs his completley in line.

I am happy with getting it bored offset bored but I would need to know more information on this.

Thanks in advance

[Sam Walters]

Every bore 0.38mm outwards from centre and 0.25mm towards the cam side of the block

Edited by Sam Walters, 21 April 2013 - 02:13 PM.

[sonikk4]

PM ACDodd as he is the man to ask.

[Sam Walters]

He may be known as an expert in mini circles but the man hones engines by hand.

Each to their own.

[Sam Walters]

The reason for the offset is so it can be bored again to 74mm. Without the offset even with only the 73.5mm bore causes head gasket problems......

Forget to put that in there. But this is bang on. I think your friend may have just been lucky with his engine.

[Sputnik]

Safe way is to use the head gasket as a guide in which direction, clearing the gasket, you should not have head gasket problems.

Edited by Sputnik, 21 April 2013 - 04:32 PM.

[Oggyp]

Would anyone have a diagram they could post up to clarify the off set boring?

[Oggyp]

And thanks for the posts already guys.

[ACDodd]

I re-space mine by measuring the actual offset in the block then adjusting this such that after boring the space between the bores at the finished size is the same.
As for hand honing, I built a reputaton on mini engines and have only ever used hand honed finishes. I even had one guy with a turbo vauxhall say he had never seen such low cylinder leakage and he also saw more lbft/pound of boost than any other build he had completed. You have to remember a machine is only as good as the operator, the trouble is I can finish a bore usually better by hand than most production machine operators, working on the A-series. Simply because I have the tigh to allow for the affects of temp distortion etc. How do I now this? I simply check the bore diameter and finish on others machining, tapers of 0.0015" are common, as re out of roundeness figures of around 0.001" they also have a tendancy to leave bore clearances tight. It is even possible to tell who has honed an a-series simply by the tolerance and shape produced. Most machinists leave a 'fingerprint' interms of the style and finished tolerances.


AC

Edited by ACDodd, 22 April 2013 - 06:27 AM.

[KernowCooper]

Just a thought here, I know you have the pistons already but would it not be better to see if the block can be bored to 1293 or 1310 before jumping all the way to 1380? once there its a resleeve back to 1275. The bhp increase would be very marginal with a slight increase in torque.

[johnnyblaze]

yehp, hafta agree with you dave, id bore the block to 1293 , 1310 the smallest that it cleans up at sell the 73.5mm pistons and buy what ever size you need, if you build your motor right sit down work out a great combination you will end up with great power , id rather leave a few more rebores up my sleeve than to stuff it and it needs a re-sleeve.

kerrin

[Cooperman]

yehp, hafta agree with you dave, id bore the block to 1293 , 1310 the smallest that it cleans up at sell the 73.5mm pistons and buy what ever size you need, if you build your motor right sit down work out a great combination you will end up with great power , id rather leave a few more rebores up my sleeve than to stuff it and it needs a re-sleeve.

kerrin

+1 more on that.

[minilee94]

Any good machine shop should know how much