325 Cylinder head on 250 Model 49?

[rootsman2]

Hi, I have read that the Model 49 325 is 'basically' a 325 'bored out'. No doubt this is an exaggerated statement, but is it possible to 'upgrade' a 250 bottom end with a 325 cylinder & piston? 

[model80]

The Model 49 Sherpa is the old style engine developed well before the 325cc engine. The first 325cc Sherpa engine came two models later with the M92.

I don't think you would have any major difficulties dropping a complete 325 engine into a M49 frame. However converting a 250cc into a 325cc without dramatically alterating the crankcases, upgrading the crank assembly, bigger exhaust pipe etc, would prove impossible to my mind.

Edited February 6, 2018 by model80

[peterb]

I am pretty sure that the 250 has a smaller crankcase mouth than the 325, and that the 250 has a smaller BCD for the cylinder bolts so,  fairly sure the swap will not work.

Bye, Peter B.

[bult360]

Hi, yes this conversion is not straight forward at all , stud centres not the same etc . Best way would be to source a model 92/125/151 etc and it will go straight in your chassis , you have to adapt the exhaust flange as the 49 has a screw in collar . The only models you can swap top end capacities are the very late ones pursang/astro etc  where both 250 and 370 shared the same 64 mm stroke , they did a special kit called kit  IT164

[jc2]

Some dealers back in the day took 125/175/250 Bultacos out to 325 using a new liner but it was a hec of a lot of work - press out the old liner, bore cylinder to take new 325 liner, open up the transfer ducts, split the cases to enlarge the crankcase 'mouth' for the new liner, machine head & combustion chamber adjusting dia & C.R. to suit larger capacity, then fit bushes on the 16mm gudgeon pin (of the smaller engines) to take the 325 piston (which used 20mm pin).

Bert Flood sold the kits in Australia & a local rider had it done on his Sherpa S 125 by our local dealer. But it always seemed a somewhat less than satisfactory practice to me, especially bushing the 16mm pin to take the 83mm piston. And if you didn't get the transfer ducts and head mods right....

 

These days guys seem to prefer boring out the 250  to take 76mm GasGas piston, giving extra 28cc. Don't know if they just bore the std liner or make new one. The Bul 250 liners are 4mm thick so that would only leave a 2mm thick liner if they just bored the liner. For me that'd be right on the limit.  Don't know if the deck ht is correct or if you need to run a reedvalve with the GG piston either (ie if there's holes in the skirt.)

 

Another idea I've toyed with but never fully investigated is to  use a 75mm KawJS550 watercraft piston, up to 1990 model which were pistonport engines (later models were reedvalve). Wiseco list them as having 16mm gudgeon, same as 250 Bul, but don't know if deck ht or ring locating pins are suitable. That'd give 21cc extra & more meat in the liner

[feetupfun]

4 hours ago, jc2 said:

These days guys seem to prefer boring out the 250  to take 76mm GasGas piston, giving extra 28cc. Don't know if they just bore the std liner or make new one. The Bul 250 liners are 4mm thick so that would only leave a 2mm thick liner if they just bored the liner. For me that'd be right on the limit.

Just thinking, a 2mm liner makes for a nicely lightweight cylinder assembly although I see the latest thing in old bikes is to use a coated aluminium liner

[section swept]

If I do my math 250 + 28 cc= 278cc whereas 250 + 75= 325. So I doubt there is much difference between a 250 and 278 but there is a noticeable margin with a 325. 28cc increase might give you 1.5 bhp extra but 75cc would possibly net you an extra 3-5 bhp. Going to specification wether it’s 238 or 244cc then the margin is even more. 325 engine uses same mount configuration as 250, so straight forward swap. You spend more cash ‘big boring. 250 to 280ish cc for not a lot of gain, probably better to get a 325 or larger engine but be careful the exhaust port/front pipe fixings may well be different. Anything is possible if you throw enough cash into the mix! There’s a degree of satisfaction in altering an engine, frame , etc because you can if you want to, so enjoy whatever option you decide upon and don’t be afraid to change your ideas at any time????. You may not get the money invested back when you come to sell etc. A 250 with a good quality re-bore and top quality piston assembly and some careful assembly and piston port matching plus ignition and carb set up should produce the best option and one which might increase the value of your machine, now that would be satisfying???

[feetupfun]

3 hours ago, section swept said:

If I do my math 250 + 28 cc= 278cc whereas 250 + 75= 325. So I doubt there is much difference between a 250 and 278 but there is a noticeable margin with a 325. 28cc increase might give you 1.5 bhp extra but 75cc would possibly net you an extra 3-5 bhp. Going to specification wether it’s 238 or 244cc then the margin is even more. 325 engine uses same mount configuration as 250, so straight forward swap. You spend more cash ‘big boring. 250 to 280ish cc for not a lot of gain, probably better to get a 325 or larger engine but be careful the exhaust port/front pipe fixings may well be different. Anything is possible if you throw enough cash into the mix! There’s a degree of satisfaction in altering an engine, frame , etc because you can if you want to, so enjoy whatever option you decide upon and don’t be afraid to change your ideas at any time????. You may not get the money invested back when you come to sell etc. A 250 with a good quality re-bore and top quality piston assembly and some careful assembly and piston port matching plus ignition and carb set up should produce the best option and one which might increase the value of your machine, now that would be satisfying???

More is not always better (280 vs 325)

If a 250 needs a rebore, it would not cost anything extra to make it 280 if you do it at the time