Is there a downside to going with a Big Bore kit?

My 70 bonny has the older style cylinders and needs a bit of freshening up so I wondered if a big bore kit is a good option. The bottom end is reasonably tight and I will rebuild the head. By the time I buy a used correct cylinder and bore it, I am only a couple of hundred away from the big bore kit. An thoughts and/or experiences would be greatly appreciated. Jim

The old Routt ones were pretty good & may be available again , also theres a nice alloy T100 P.U. ish one , A.R.E. ? to save weight .
Biggest bug bear would be boreing out the crankcase mouths and if them screws still go in between the cams .



Also some people think 40 inch is the size for twins , parr tickue larr ly if your going to rev the snot out of it . Brings us to PISTON WEIGHT .

Depend on your use , if its all twisty back roads solo & you dont mind the odd gear shift , 650s more than adequate .

Two Up Touring interstate , the extra tourque wouldnt do any harm with the bigger pistones .

There are some great kits available, mainly in US, that use high quality, lightweight forged pistons. They'll be lighter and stronger than the stock pistons. IMHO, there's no reason NOT to to for a bigger bore in your situation!

I put a big bore kit on my '74 Bonnie back in about 1984. Rode it off and on for another 10 years. Didn't really cause any problems, but neither was there an appreciable power increase. Probably a bit more low end torque, but not as free-revving as stock. Assuming equally good quality parts either way, I'd leave it stock. If you're looking for a big power bump, I think you'll be disappointed.

Matt, what do you mean exactly when you say,

"Biggest bug bear would be boreing out the crankcase mouths and if them screws still go in between the cams ."

Morgo barrels seem to be OK. I had an Alfin set years ago. They consisted of cast iron liners with bosses for the head studs, cast into a set of aluminium fins. Cracks appeared right through the centres of a few of the threaded stud holes. I thought of bronze welding the cast iron stud holes, then gave up after it had cost me a bomb re-sleeving them and stuffing around.

I rode a 71 Bonnie with a Morgo kit for 15 seasons, and I've owned and overhauled and ridden a bunch of Triumphs before, after and in between. If installed correctly a 750 kitted engine will rev out as quickly as a 650 with little penalty in vibrations but with a nice hike in power and torque. The big inlet ports and valves suits the larger volume better, for increased inlet gas velocity at lower rpms. Throttle response becomes better too.
I have however sampled big bore bikes that used to be sweet and quick as standard but became lemons with bigger cylinders.
So yes there can be downsides, but more likely you'll be happy if you go that route IMHO.
Still, a good 650 is fun and all most people will ever need, also IMHO...

Go to Triplesrule.com and check out the new kits patrick brown has to offer with nilkasil coated bores. I'm waiting on my kit to arrive any day now.