Sherpa T Engine Development

[hoodie2]

Hi all. Had a notion to build myself an early 70s sherpaT for nostalgic reasons. First bike I fell for was a model 91 Sherpa T when I was 8. Always wanted one and finally got myself a an m92 frame. This is the starting point for a long term project. I was wondering how the engines vary accross the 70s and what range of engine numbers I should be looking for to get one that is compatible with this frame. Any guidance would be much appreciated.

Cheers.

Hoodie

[woody]

They didn't change much really. The early 325 motor from the model 92 to 151 (1972 - 75) was basically an overbored 250 barrel with square fins and these have a limited oversize due to the thinner liner. From 1975/76 they had internal changes to the crank, gear ratios, selectors, porting maybe, but nothing major. These engines had thicker liners and could be bored out to 340cc with the Pursang piston which you can't do on the earlier ones. Not that the 325 needs a performance hike anyway.

Visually they are pretty much the same so you could use any of the 325 motors. Clutch cases changed design in about 1975, model 151 maybe, and the timing cover changed in 1979 on the 199a. However for appearance sake the older cases can be fitted to the later engine if you're bothered. The fin design on the cylinders/heads differs slightly from older to newer but it's not obvious. The later 325 engine has a larger exhaust port flange than the earlier engine so the exhausts aren't interchangeable. The earlier 325 uses the same size as the square fin barrel 250 I think. Earlier engine had an Amal, later used a Bing, these take different inlet manifolds but they are interchangeable. The most noticeable change in appearance was to exhaust systems, silencers in particular.

The 250 engine changed from round finned barrel to the square fin 325 style in 1975 so the 250 from that date differs in appearance from an earlier engine. Clutch/timing cover as per 325. Early 250 uses Amal mk1, 1975 on uses Amal mk2. Inlet manifolds differ but are interchangeable. Exhaust design completely different from round to square barrel.

For your bike, if you want to keep it a 325 to go with the 92 frame, apart from purists knowing the differences, any 325 motor would look ok with the older casings, even the later clutch cover, but you might not be able to get a 'period' appearance exhaust if you use a later 325 motor. You're stuck with the later exaust, unless you modify. If you use a 250 motor then you really need the round barrel model for that year frame.

Ultimately, all you want is a bike to ride and enjoy so if it was me I'd use any engine I could get, none will look out of place really. There were plenty of hybrids back in their era. Originality can become obsessive and ultimately, for what purpose.

[hoodie2]

Thanks Woody. That's been a big help. I've got a much better idea what I'm looking for now. On the 325; When the internal changes to crank etc. came in in 75/76 did the cases change too or will those cranks gearboxes fit older cases?

Thanks again for the help.

Cheers.

[woody]

I can't say this as definite fact but as far as I know the cranks would be the same in terms of external dimensions so would fit any 325 engine, I think they were just 'beefed up'. I don't believe there were any changes to the cases. You'd be better checking with one of the Bulto specialists on this though, just to be sure

One thing I forgot to mention is that there were changes to the clutch hub around the early 70s and this could cause you trouble if building an engine from sourced parts as it affects the gearbox mainshaft too.

The 250 model type 49 (and 80 I think) had woodfruff key fitting for the clutch hub on the gearbox mainshaft. Sometime after that they changed the hub and shaft to a spline fitting. This may have happened on the model 91 but the next model, 124, definitely has it. You can't put the splined mainshaft/clutch hub in the earlier engine as the shaft is a larger diameter, therefore the bearings are different sizes, ID and OD. Naturally, same applies to putting earlier shaft/hub in later engine.

All 325 engines will have the splined hub.

The other thing to watch with the hub is that the boss on the rear which fits up against the spacer is different depending which bike it has come from. There are three lengths that I know of and if the wrong one is fitted, could throw the primary chain out of line (I'm assuming) There is the woodfruff key type with nuts to hold the springs. There is a splined type as fitted to the 92 with pins to hold the springs. There is a splined type with nuts to hold the springs as fitted to all of the later bikes. All have differing lengths. Best option is to get an engine with the clutch complete, then you don't have to worry. If you start with a pair of bare crankcases and try to build an engine from there, you may just rip your hair out trying to match clutch parts....

[hoodie2]

Hi Woody and anyone else who wants to chip in! I

[woody]

The problem with those parts is you're going to have real trouble identifying which model they have come from, could even be several, so establishing which cases they will fit in order to buy some and whether a gearbox is compatible with the clutch will be virtually impossible - until you come to assemble it all. Also, a 73mm piston is from a 250, not a 325 which is 10mm bigger in diameter. The clutch chain tensioner should have a revolving wheel, not a plastic triangular pad.

If it was me, I'd look out for a complete engine as at least you know what you are starting with. If parts are missing/worn, at least you know which model it is when ordering replacement parts. They come up on ebay from time to time and Ellastone Offroad near Uttoxeter has Bultaco stuff and may be worth a try for an engine to rebuild. They are also going to the USA soon and may well be bringing more Bultaco stuff back. Lots of engines being broken for parts there (I'm assuming you're in the UK)

Other issues keep coming to mind as well each time I think about it. The later clutch-side crankshaft weight is a different shape on later models and won't fit inside the earlier clutch case (up to '74) they need some metal machining off the outside edge in order to clear the inside of an earlier case. Gear selectors changed around '74 also, They got beefed up a bit and I don't think the early selectors will work with a later gearbox and vice-versa. Most of these parts look identical when looking at them individually and strewn over a bench but when you compare them side by side you notice the differences. So although nothing was a major redesign throughout the life of the Sherpa engine there are enough little differences on engines up to around '75 to make life very difficult to do what you propose. After that I'd guess they stayed pretty static.

Try and get a complete engine, it's the better option

[hoodie2]

Thanks Woody. I spoke with Pete at Bultaco uk and he was of the same opinion. Looks like I'm looking for an engine or at least a bottom end for a 250 or a 325 from 1971-1975. If anybody has anything they are getting rid of that might suit could you let me know by PM.

Cheers and thanks again for the advice.

[hoodie2]

Just to finish this thread; thanks to this forum I got an offer of an M92 engine, that has a number only 168 from the frame number, which I'm very grateful for. Looks like the project's on now.

Cheers

[rootsman2]

I'm looking for a 325/350 jug and piston set to fit my 250 or a complete engine if anybody knows anything - I think my frame  and engine are from 1970. I think.

[lbhbul]

Hi & welcome to TC

 

I don't believe you can just use a 325 jug and the rest on a 250 bottom end as the bore in the case is larger, ref 72mm bore 250,

and 83.2 bore 325.

M92 thru m151 did use a modified 250 case but a different crank which has a larger crank pin & top end bearing, so you really need to

stay@ 250 or find a real 325 engine.

 

Sorry to pass on this news.

 

What are the engine numbers you now have?

 

Larry