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I am working my way through a Narrow Case 250...probably used to be a Monza...who cares? So, the engine will be coming apart and I'm pretty confident that I will want a new piston & rings. While its apart...and I have the chance to bore out the cases if necessary...can anyone suggest a way forward to increase the displacement? How big of a liner can I push in? What kind of piston can I use to increase the displacement?

Obviously the most "correct" and easiest thing to do is put in a Mach 1 piston and rings and one of the performance cams...may be some larger valves, etc...But what I really want is just a bit more displacement and a relatively soft power band.

Any ideas???

Thanks in advance

Mike

Hi Mike,
The largest bore you can accommodate in the stock 250 liner is 77mm. The 250 liner is 79.8mm o.d. The 350 liner is 81mm o.d. You could certainly bore the cases to 81mm since the 250 and 350 cases are basically the same. There is probably room for 1-2mm more but to go beyond 77mm you’re looking at custom made pistons. 77mm pistons are available but compression ratio is probably higher than you would want, making staring difficult, high octane gasoline necessary and temperamental tuning. For your stated goals with readily available parts, you might consider a 350 scrambler piston and scrambler cam. The available pistons likely have valve reliefs for 40mm intake/36mm exhaust valves. Your Monza head has 37mm intake and 33mm exhaust valves. You would have to consider what you want to undertake as far as valve sizes, carburetor and porting to get everything working in harmony. Stock Monza carb is 24mm so I expect you would want to add a larger carb. The Monza head is a good starting point as long as it hasn’t already been tampered with but the ports are unsuitable for your objective. There are a number of places that can produce a cylinder head suitable for your objectives.

Matt

hi mike

i am taking my narrowcase out to 372 cc using a 350 Sebring crankshaft and barrel .
had LA sleeve made me a bigbore liner with 84 mm o.d.
custom made J/E piston in 79.50 mm .

for the long distance racers Ducati build some 285 cc motors in the 60's

Eldert

Many thanks for the replies. Phil, down in the Southern Hemisphere, suggested simply using a Mach 1 piston and sticking in some oversize valves with a scrambler cam. This seems easier and I'm certainly not trying to win any races with this old Italian...but I would like it to be a bit faster than my BSA B25.

May be I'll look for a complete 350 or 450 after I finish this project. I won't have to worry about sourcing a tach, and a correct narrow front fender, and those unobtainium rear sets.

Thanks again...

Mike