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+40 thou rebore

40thou of 1 inch equals roughly what in mm (no calc to hand) then take the bore size, add it on, using the volume for a cylinder you can work out the new volume, then multiply by 4... etc

have no calc to hand unfort....

put it like tihs it won't be much, probably from 2235 to 2240 cc not really worth it.

better off putting on a decent electronic ignition and trying to raise the compression ratio...
 
Thanks for your reply

My engine has already been rebored and i just wanted to know how it differs from original

Thanks fo your help

SCHREIBER
 
I remember some guy here in the states took a new 2.25 and bored it +40 or +60 to try to get more performance, Added a 2.5 cam, ported the head, took .1" off to boost the compression to 9:1 or so. Added a Weber 2bbl with Pierce intake, headers etc. In the end it WAS faster but he said it really was not worth all the $ and effort. It would have been easier to adapt another engine if he'd wanted significantly more power.
 
Bore size is 9.047 cm as standard, with a cross sectional area of 28.422 cm sq.

An 0.004 inch oversize is an additional 0.010 cm, so the bore size is now 9.057 cm. The new cross sectional area is 28.453 cm sq.

By ratio the new capacity is 2286 * (28.453 / 28.422) = 2288.5 cc.

Conclusion? Increase the bore when the cylinder walls become worn; not for additional power. You can only go so far before cylinder liners are required.

Jonathan Paton

Disclaimer: I've used the bore size and the engine capacity figures without knowing the geometry of the engine cylinder/head. I've never taken an engine to bits so I'm not an expert of any sort. Math/calculator mistakes are possible.
 
Bore size is 9.047 cm as standard, with a cross sectional area of 28.422 cm sq.

An 0.004 inch oversize is an additional 0.010 cm, so the bore size is now 9.057 cm. The new cross sectional area is 28.453 cm sq.

By ratio the new capacity is 2286 * (28.453 / 28.422) = 2288.5 cc.

Conclusion? Increase the bore when the cylinder walls become worn; not for additional power. You can only go so far before cylinder liners are required.

Jonathan Paton

Disclaimer: I've used the bore size and the engine capacity figures without knowing the geometry of the engine cylinder/head. I've never taken an engine to bits so I'm not an expert of any sort. Math/calculator mistakes are possible.

Bless this man for sticking his heed out
 
40 thou is the maximum oversize piston and rebore. Once the bores need reworking again, they will have to be relined. The size will have no effect on power or torque - 40 thou is about the thickness of a hair.
 
Make shure your pistons have 2 oil rings, one at the bottom end and one in the middle. Otherwhise they won´t compress decently and blow air through the valve rocker cover and in the scoop and cause some white smoke.
It happened to me after ca. 200 km run on the engine (.40 rebore) My pistons didn´t come with the bottom end oil rings!

sjunni
 
40 thou is the maximum oversize piston and rebore. Once the bores need reworking again, they will have to be relined. The size will have no effect on power or torque - 40 thou is about the thickness of a hair.


My 2.25 diesel engine is overbored to 60 thou which I think is the max. I didn't do it. Takes wear out but its not a worthwhile performace upgrade. Cheaper and more effective to get a more powerful engine.
 
My 2.25 diesel engine is overbored to 60 thou which I think is the max. I didn't do it. Takes wear out but its not a worthwhile performace upgrade. Cheaper and more effective to get a more powerful engine.
Perhaps for the 2.25. I'm pretty certain that +40 is the max for 2.5s.
 
I was pretty sure that Performance build I was referring to was +60. I remember people commenting about how he had wasted a perfectly good block inthe process.

That build was a custom one- They used Ford Valve springs and I almos think pistons from a different application.
 
Hi there,
I thought 40 thou = 1mm
I was always taught to leave as much ' meat ' in the liners as you can get away with , i.e. - only rebore to 10 thou if that's all thats needed.
If you think you will feel a performance difference between a 40 thou rebore and a 10 thou rebore it may be a case of wishful thinking !
;) Joe
 
I don't think it is generally worth it with a 4 cylinder. It is a very common way of upgrading displacement in US 8 cylinders but naturally that is a different subject.

Naturally if you are boring to correct problems in the bore you want to go as little over sized as is necessary- that is why it was such a waste to take a new block and blow it 60 over in the engine I was talking about. The only way to reclaim that once it wore out would be to sleeve it.:(

For your average rebuild a .40 over might give some theoretical advantage in displacement but I doubt a seat of the pants test would be able to detect the difference! :D
 
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