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1275 Confusion


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#1 Will96

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Posted 05 January 2019 - 07:31 PM

Hiya guys, I'm in the process of reconditioned a "1275" engine block I bought off of a well known bidding website. The quotations because it was advertised as a +20 rebore yet upon analysis they seem to measure 73.6mm, and my brief research has pointed out it may be the full wack 1380 oversized big lad. Just wondering if anyone could clarify my suspicion before I buy some new piston rings. And also I'm after the diameter of the main bearing seats on the crankshaft. Thanks in advance

#2 grizzler73

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Posted 05 January 2019 - 07:46 PM

What do you mean by "main bearing seats"?
Sounds like a 1380, they have 73.5mm pistons.

#3 GraemeC

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Posted 05 January 2019 - 08:01 PM

Are there no markings on the crown of the pistons?

For main bearings and big ends - take out what is there and see what’s stamped on the back. Take the crank to a machine shop and get them to look at it and advise whether it needs a polish and new bearings or a grind and new bearings.

#4 Cooperman

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Posted 05 January 2019 - 08:16 PM

Before you spend money on it make sure you measure everything.
If is a 1380 the first thing to establish is whether it needs a further re-bore. If it does then finding a block which has not been bored to 1380. Also, if it is 1380 and just needs new rings, make sure that it has been correctly offset bored.

#5 Will96

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Posted 05 January 2019 - 08:44 PM

By the seats I mean the points in which the crankshaft is sat against the engine block. Got the option of buying oversized main bearings and was just thinking of avoiding trial and error buying. But looking at previous is a good shout, didn't think of that. The guy that I bought the engine off of said that it had been running but a "gearbox fault" meant he was selling it. Now it's established that it is a 1380, so like +60 instead of the advertised +20 I'm doubtful of how much I can trust what I was told about the block.

#6 Cooperman

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Posted 05 January 2019 - 09:18 PM

A 1380 is not +0.060". That would be 1330 cc which is the maximum you can go to without offset boring. When a 1380 requires further re-boring there is a risk of scrapping the block and to re-sleeve it can also be a problem and expensive. Measure accurately what you have before doing anything else.

It may be that the bores will take a light honing and the pistons have new rings, but that does indicate that a level of bore wear has already taken place and it will not be long before a new block is needed if it is already at 1380.

It may be better to spend the money on another block within normal boring limits, i.e. +0.020" or 0.040". That will give a new bore which will have a good life followed by the ability to go a further bore up or to sleeve it back to 1275 if it ever needs a another re-bore.

There is really no need to ever go bigger than +0.060" in terms of power or torque unless it is for full competition use.



#7 nicklouse

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Posted 06 January 2019 - 01:25 PM

By the seats I mean the points in which the crankshaft is sat against the engine block. 

well that depends on what they have been ground to.

 

time to get measuring and may be regrinding if the gearbox issue has caused crap to go through the bearings.



#8 Moke Spider

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Posted 06 January 2019 - 05:20 PM

It appears you've not had much experience with the finer items of engine internals.

 

To save yourself some frustration, and time, I'd suggest having it all looked at & measured up by a Machine Shop before you go too far.

 

It maybe possible if the bores are worn to have the block sleeved. I feel this is a good option as you can likely bring it back to standard sized bores and the material the sleeves are made from is way better than the stock bores.

 

As Nick touched on, what ever bearings you need for the crankshaft will be dependent upon what size that is presently at and it's condition, it made need grinding to get an acceptable size, shape and finish.

 

There are loads of other items to look at beyond what's been discussed here and I really feel you need someone with an  experienced eye to go over it and provide their words of wisdom. Depending on where you are, I think AC Dodd (on this forum) can offer this service, but there is also others.



#9 ACDodd

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Posted 06 January 2019 - 09:13 PM

I provide a measuring up service for forum members and it's free!

I have only ever done it 3 times in all the times I have offered it. So you can see how much people don't realise how important measuring up is!!

Ac

#10 viz139

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Posted 07 January 2019 - 11:18 AM

Since it was unknown that the engine was overbored to 1380 you can't rule out that it may also be stroked with cooper S rods and smaller bearings. You may have hit lucky with your purchase and investing in a proper engine builder may reap great rewards.






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