Crankcase question

Is it possible to use 99 crankcases for an 88 engine?

Definitely.......apart from the engine and shop numbers stamped all around the sides, the basic cases are generally identical. eg same casting numbers. They both use the same size main and big end bearings. This also applies to model 7 and 77 cases. (& Nomad and 99SS)

The points to check are whether you need the cases for a dynamo or alternator engine. Then, there may be a need to check the primary chaincase as they have different mountings.

You could even use 650 cases as the studs for the barrels had the same positions as on 88 & 99 engines.

You can bolt a 99 head on to 88 barrels. The pre 1961 versions back to the iron heads have identical mounting hole positions. Though some used a different size carb manifold.

I think the only parts that can't be freelyswopped between motors are the cranks,barrels ,pistons and pushrods.There are plenty of 600 cc 88's and 500cc 99's.There is also a 750 88 (with a bit more work!)The conrods are also common and seem to give no trouble in the 88 but are less happy in the 99's.

You can even put 99 barrels and pistons on an 88. The compression ratio is fairly low but it does make a very sweet running motor.

Never thought of doing that,but it sounds like my sort of bike. If i did that with my 60 99 which has 650 ss pistons what would i finish up with? in the way of capacity and compression.My crank has got to be overhauled so slotting in an 88 crank would be simple ,but it would need a re-balance,and a light hone possibly at the bottom of the stroke.Should make for a smoother revvy motor with perhaps a pw3 cam.

You can also rebore an 88 barrel beyond +60" and then fit standard size 99 or 650 pistons. Somewhere around +80" or to 67.995mm I believe. I guess a crankshaft rebalance might possibly be a good move.

The capacity must increase to around 535cc. I wonder what an insurance company would make of this mod?

I have bored an 88 to 68mm and fitted 650 pistons and had no need to rebalance - it was a smooth motor before and after. Sometimes you are lucky!

As far as I can tell, the 88 and 99 barrel castings start off with the same wall thickness, so the 88 being 66mm bore has1mm thicker walls than the 99 at 68mm bore. This explains why when a rod breaks with an 88, usually the barrels survive but in the case of a 99 the barrels are usually smashed, the walls being thinner and weaker. Boring a 99 barrel beyond +040" is very much at your own risk.

Personally, whileI was happy enough tohave the 88 barrels bored to 68mm and fitting 650 pistons, I wouldn't take it any further without re-sleeving, though theoretically you could goas far as +.040.

As for the sleeves, I don't have the dimensions unfortunately.Presumably there will be variations depending on who manufactures them.

You can also rebore an 88 barrel beyond +60" and then fit standard size 99 or 650 pistons. Somewhere around +80" or to 67.995mm I believe. I guess a crankshaft rebalance might possibly be a good move.

The capacity must increase to around 535cc. I wonder what an insurance company would make of this mod?

well phil if it say model 88 on the log book I would not tell any one any different , yours anna j