14 replies to this topic

[gazza82]

Are these all the same ie is the centre of each bore in the same place on all engines? I'm assuming these are normal and not been offset rebores.

[nicklouse]

are you meaning across the large and small bore blocks?

[Cooperman]

All 1275 engines have the same distances between bore centres and all the 850, 998 & 1098 engines have the same distances between bore centres. The 970 and 1071 Cooper 'S' engines have the same distances as the 1275.

[gazza82]

Many thanks. It was one of those silly niggling questions you get sometimes ... I assumed they were but just needed it confirmed so I could sleep again ... or go and find one of each with a vernier handy!

[gazza82]

A fellow A35 owner found this!

- 998cc A-Series (64.58mm bore): cyl.1&2/3&4: 5.5mm, 2&3: 18mm
- 1275cc A-Series (70.66mm bore): 6.5mm

The smal-bore engines (848, 948, 998, 1098) have paired bores, hence a bigger spacing between 2 and 3. The big bore engines have even spacing (970, 1071, 1275). P.e. 848cc vs. 1275cc:

[tiger99]

And, anything over 1275, eg 1380, has the bores offset both laterally and fore and aft. That is not good practice for a number of reasons but is the only way to get the capacity.

[Sprocket]

All 1275 engines have the same distances between bore centres and all the 850, 998 & 1098 engines have the same distances between bore centres. The 970 and 1071 Cooper 'S' engines have the same distances as the 1275.

On the large bore blocks, cylinders 1/2 and 3/4 have the same centre distances, but cylinders 2/3 have a slightly smaller centre distance. It is for this reason Offset boring is required on the larger over bores to ensure a reliable gasket seal. You are basically increasing the thickness of the cylinder wall between 2/3 by 0.5mm in a correctly offset bored block.

[Sprocket]

And, anything over 1275, eg 1380, has the bores offset both laterally and fore and aft. That is not good practice for a number of reasons but is the only way to get the capacity.

The 1380 and 1399 bores only need offset boring to stretch the centre between 2/3. you are simply moving 1/2 and 3/4 away from each other by 0.010" each pair (0.020" increase in centre distance on 2/3). Nobody ever breaks through into the oil galleries.

 

If you feel that inclined to also introduce an offset to move all the cylinders towards the tappet chest, by all means go ahead, but all you are doing is introducing more friction in the cylinders by having the cylinder centres over the crankshaft on a skew, increasing the friction on the piston skirts. It might be marginal, and maybe un-measurable on a dyno, but the wear on the parts will be higher. it is not necessary.

Edited by Sprocket, 19 April 2017 - 11:34 PM.

[Cooperman]

 

All 1275 engines have the same distances between bore centres and all the 850, 998 & 1098 engines have the same distances between bore centres. The 970 and 1071 Cooper 'S' engines have the same distances as the 1275.

On the large bore blocks, cylinders 1/2 and 3/4 have the same centre distances, but cylinders 2/3 have a slightly smaller centre distance. It is for this reason Offset boring is required on the larger over bores to ensure a reliable gasket seal. You are basically increasing the thickness of the cylinder wall between 2/3 by 0.5mm in a correctly offset bored block.

 

What I meant is that all 1275 engines, plus the 970 'S' & 1071 'S' have the same cylinder positions, but the 850/997/998 & 1098 are different from the 'big bore' units, but the same as each other, if that makes sense.

 

I have heard of offset boring 2 & 3 both in-line and slightly offset. If this is done 1 & 4 must also be offset laterally to make piston TDC height the same for each cylinder which is not a good idea, One problem with offset-boring in-line to increase the clearance between 2 & 3 is that the cylinder walls can become very thin, depending on the original casting tolerances and it can cause problems, if not at the bore to 1380, but when a further re-bore is needed. It is true that in many cases special sleeves can be fitted and the block bored again from there, but it is expensive and a block can be scrap once it gets to the next bore after 1380. We need to preserve these classic engines as there will never be any more at affordable prices.

 

For the very small gain in going to 1380 it is arguably not worth doing.

[Sprocket]

The block is already too far gone if a 73.5mm bore needs extra work, but then how often does anybody rebore a block never mind it being 1380? Just not worth the bother when blocks are so easy to come by (considering A+)

74mm is an option but not at all very common these days.

The bore thickness on the A+ blocks wont be a problem. Any of the really early blocks i'd be causious though.

Anyone considering boring a Cooper S block these days to 73.5mm seriously needs their head looking at. IMO.

Edited by Sprocket, 20 April 2017 - 08:35 PM.

[tiger99]

Personally I would not want to take a block to 1380. It would seem to be sensible to sleeve it back to 1275 if it has had too many rebores instead of offsetting the bores, which practice apparently is quite common. It will be more reliable at 1275. The big advantage of sleeving is that if a modern material is used the sleeves will last 3 or 4 times as long as a standard cylinder.

But on the subject of bore centres, how well does the BMW bike head conversion actually match either a 1275 or a 998? It can't be a perfect match to both. How much mismatch is acceptable? And what about gaskets? Just curious...

[Cooperman]

As above, there is always some risk in going to 1380 and as well as being expensive to offset bore compared with standard boring, does mean problems if a further bore is needed later because you can't fit standard sleeves into a 1380.

 

I had a 1380 brought to me a while back. The owner had had the engine built and bored to 1380, but as the car was not finished until almost 2 years after the engine was built, the warranty on the engine had expired. When he came to run it there was a big problem. It was throwing oil out of the rocker cover and smoking badly.

 

Eventually when I got it and measured it the bores were an average of 0.004" oversize, so the rings were never going to be able to control the oil. It had not been offset bored either! The options were to sleeve it back and make it 1275 again and buy new pistons, or to get another block and bore it to 1380. We chose to get a replacement block and offset bore it for 1380 and it was OK, although I didn't think it was even as good as a well-built 1275. The head work was non-existent to poor and the cam choice was not ideal either. The extra costs of a really good head and a more suitable cam would have been a better use of the money spent.

 

I think the owner wanted a 1380 so that he could brag about it to other Mini owners.

[tiger99]

I think the owner wanted a 1380 so that he could brag about it to other Mini owners.

 

Almost every time I see mention of a 1380 I have the same thought!

[Sprocket]

Personally I would not want to take a block to 1380. It would seem to be sensible to sleeve it back to 1275 if it has had too many rebores instead of offsetting the bores, which practice apparently is quite common. It will be more reliable at 1275. The big advantage of sleeving is that if a modern material is used the sleeves will last 3 or 4 times as long as a standard cylinder.

But on the subject of bore centres, how well does the BMW bike head conversion actually match either a 1275 or a 998? It can't be a perfect match to both. How much mismatch is acceptable? And what about gaskets? Just curious...

 

 

All depends who builds it

 

In my case, I offset bored the cylinders and installed liners to take it back to 1275cc. The cylinder head fits my block as if it was the BMW motorcycle block and I use the BMW head gasket Something no one else has ever done to my knowledge...............

[tiger99]

So you offset bored the cylinders to match the BMW head properly? That is one place where offset boring really does make sense.