9 replies to this topic

[danrock101]

I'm going to be using omega 73.5mm pistons in my next engine, philc had a block for sale on here at a lot cheaper price than a rebore would cost. So I thought if I bought that and a bought a hand hone tool to give it a quick lick up do you think my omegas would fit OK. He used 73.5m omegas in it before hand, I don't know if they're all exact or what? I can't see why not, they're both cast

[Cooperman]

I'm going to be using omega 73.5mm pistons in my next engine, philc had a block for sale on here at a lot cheaper price than a rebore would cost. So I thought if I bought that and a bought a hand hone tool to give it a quick lick up do you think my omegas would fit OK. He used 73.5m omegas in it before hand, I don't know if they're all exact or what? I can't see why not, they're both cast

Modern CNC manufacturing technology gives superb accuracy and repeatability in volume production both dimensionally and weight-wise.
The question is whether the bore which you intend to hone is accurately machined and does not suffer from excessive wear at mid stroke and has no 'step'at the top of the top ring's stroke.
A 1380 block should be 'offset' bored to increase the centre-to-centre bore dimension from cyl 2 to cyl 3. The offset should be around 0.030" in total, i.e. 0.015 for each cylinder.

[danrock101]

I assume it's offset but I'd have to check, wouldn't buy it if it wasn't, glad to hear the pistons should be same size, will have to check the bore wear. May save me a few ££ if it good, cheers!

[MRA]

Let me just clarify this ....

You intend to fit very expensive brand new Omega pistons in an old used block ?

It really is a recipe for disaster even if the tolerance on the day that both sets were produced is identical unless the block was bored / honed undersized then you will need to hone it for the new pistons/rings which will leave it oversize, so your saving will reduce the life of your engine by a considerable amount, whilst also increasing blow by of the rings and a lower power, higher oil consumption, etc......

It really is throwing good money after bad

[danrock101]

hmm that was the idea but I was more wondering, just thought for £50, then sell my old 1275 block for that it would be a much needed saving but not looking good if it wouldn't last long obviously and I havn't got much time to spare so it would save me a lot of time IF it would work. Wasn't too sure how much honing you can do before you need to go up a piston size either

[Cooperman]

hmm that was the idea but I was more wondering, just thought for £50, then sell my old 1275 block for that it would be a much needed saving but not looking good if it wouldn't last long obviously and I havn't got much time to spare so it would save me a lot of time IF it would work. Wasn't too sure how much honing you can do before you need to go up a piston size either

As the block is 1380 now, there is no guarantee that it'll go to the next size up without going through the water jacket. Of course, it might well be OK, a lot are, but there are some which aren't. If you try and it fails, you still have to pay for the boring.
To be quite honest, you would probably be better going for a block which has not been re-bored much and taking it out to +0.060", i.e. 1330 cc, and using 21253 pistons. Then you have some more to bore later and won't have to worry about it.

[danrock101]

I've already got some 73.5mm omegas fitted to my aeg 521 rods to be used with the long stroke crank and if I went a lower bore size it would cause a few problems realy, another reason for thinking about getting a ready bored block is to not machine the life out of another block (admitadly the main motive was to save money & time) but I'm not sure what to do now lol. Not a good idea to bother with the used block by the sound of it lol

Edited by danrock101, 13 April 2011 - 02:21 AM.

[Cooperman]

I've already got some 73.5mm omegas fitted to my aeg 521 rods to be used with the long stroke crank and if I went a lower bore size it would cause a few problems realy, another reason for thinking about getting a ready bored block is to not machine the life out of another block (admitadly the main motive was to save money & time) but I'm not sure what to do now lol. Not a good idea to bother with the used block by the sound of it lol

Thinking a bit laterally, if you are going for a very large capacity engine one might assume that it is because you want to really use the increased performance. Thus the engine will be worked hard and needs to be properly engineered. With a stroked crank the mean piston speeds are going to be even higher and that will also increase bore wear and show up any poor tolerances.
If you build it all up and the oil consumption is excessive after just a few miles, you will have to start again.
Personally I don't really like 1380, i.e. 73.5 mm bore, engines as that effectively scraps the block once it is worn and there will probably never be any more A-series blocks cast. A nice capacity for good all-round performance is +0.060" which is 1330 cc. Properly built there will not be much difference in absolute power, but the 1330 block can be bored again, and probably then again, or linered back to 1275 as it's not an offset bore size, which saves another block. Prices for 1275 blocks are increasing as they become less available now.

[monkey]

Sorry to hijack, but why can you not put liners into an ofset bored block to take it back to 1275?

Steve.

[MRA]

Because the strength and integrity of the block is seriously affected