9 replies to this topic

[DUF2]

Hi I’m just wondering how you get to larger capacities on a 1275 A+ engine

i know a 1293 is a 20 thou oversize bore. But how do you go for larger capacities safely?

[nicklouse]

Bore the cylinders bigger. The larger boring operations need for the boring to be offset so the gap between 2-3 does not get too thin.

[DUF2]

Bore the cylinders bigger. The larger boring operations need for the boring to be offset so the gap between 2-3 does not get too thin.

Nicklouse that is what I was wondering, because the bores on 2 & 3 are close,

so can a 1275 engine be bored 60 thou oversize?  If the bore has to be offset, how are the differences between the centres of the journal on the crank and piston accommodated?

[mini13]

The info in the big yellow Vizard book says to offset bores outwards from the centre main by 15 thou ( to maximise meat between cyls 2&3), and back towards the pushrod holes by 10 thou.

 

iirc its only really needed on 1380's, but you need to have offset earlier otherwise you cant take a pre bored block to 1380. ( i think 1330 upwards needs to have been ofset already)

 

the piston will ove on the gudeon pin to accomodate the extra, so no problems there.

[gazza82]

Hi I’m just wondering how you get to larger capacities on a 1275 A+ engine
i know a 1293 is a 20 thou oversize bore. But how do you go for larger capacities safely?

20 thou: 1293
40 thou: 1310
60 thou: 1330

[Cooperman]

Up to +60 thou, which gives 1330 cc, no offset boring is necessary.

it is possible to go larger than 1330 cc, but offest boring is needed really. Then, when a further re-bore is needed it can be very expensive to sleeve back to 1275. The best option is to over-bore the 3 times (1293, 1310 then 1330) and then, when a futher re-bore is neded, bore and sleve back to 1275 cc. That way the block can be made to last a very long time, even for a competition engine used for racing. A block for a road car would last almost forever.

[Spider]

I do agree with Peter here that for road going stuff, the bigger over bores really just aren't warranted. Yes, they will make a bit more power, but really, how often are you that hard on the loud pedal ? I've had some great results from 1293 and 1310.

 

If though you think you might be inclined to ever go to a bigger over bore, you need to have the block off set bored from it's first over-bore. The full off-set might not be achievable on the first bite, but likely on the second, so you'd want to keep good records of each re-bore.

[timmy850]

The other way to add more capacity is to use a stroker (longer stroke) crankshaft.

[DUF2]

Thanks for all the answers, I was curious how they did it.

my motor is working well enough at the moment & doesn’t need a rebore (yet!) if I did I would probably bite the bullet and go straight for a 60 thou overbore.

I built a Ford Kent engine many years ago. I used a 1500 pre crossflow block, bored to +60 thou, used a Crossflow head with with the largest valves I could get. Fitted 1100 high compression pistons on standard 105E Conrad’s to get the correct deck height and a lotus twin cam crankshaft, so ended up with a 1558cc crossflow engine.  Not sure on the BHP it produced but it would pull 8K in top gear on a 4.4 diff.

Edited by DUF2, 31 August 2021 - 11:52 AM.

[nicklouse]

Thanks for all the answers, I was curious how they did it.

my motor is working well enough at the moment & doesn’t need a rebore (yet!) if I did I would probably bite the bullet and go straight for a 60 thou overbore.

I built a Ford Kent engine many years ago. I used a 1500 pre crossflow block, bored to +60 thou, used a Crossflow head with with the largest valves I could get. Fitted 1100 high compression pistons on standard 105E Conrad’s to get the correct deck height and a lotus twin cam crankshaft, so ended up with a 1558cc crossflow engine.  Not sure on the BHP it produced but it would pull 8K in top gear on a 4.4 diff.

I would only go as big as needed. Then you can always go further as needed.