Pre-unit/UnitTriumph Engine

I've got the head,barrel, cases and crank for a pre-unit 650 and need the rest. Will, if any of the unit parts fit?

Thanks,

Cash

Lots of stuff from lots of years will fit, and can be made to work properly, but you need to be careful choosing because not ALL years/models fit.

Are the bits you have all original to the cases? If so, what are the case serial numbers? (At least as "30XXX")

Cash, try and get hold of this book - Bonnie by John Nelson. He was the former service manager for Triumph and he covers the development and every variation for every year of the Bonnie from the original 650 pre-unit to the last of the Meriden 750's. It also includes all the part numbers for each year!

The title Bonnie is misleading as much of the engine and transmission was common to all of the 650 twins. There are many parts from units that will readily fit pre-unit and others to be avoided. The detail is extensive and as GP poinst out, there are some very subtle minor differences from year to year in some parts that can make a random rebuild very exasperating.

Mick

Pre-unit 650s also include 6T and T110, not just Bonnies.

The models differed very little from year to year. Maybe one reason they are so popular still; very easily interchanged.

Thanks Chaps, and I'll try to get hold that book.

The Triton might start to roll.
Cash

It would help that it's going on a Triton, there are differences in the transmission adjusters and tranny mounting lugs that disallow certain combinations of power units & trannies in Triumph frames without cutting & welding.

cash said:

I've got the head,barrel, cases and crank for a pre-unit 650 and need the rest. Will, if any of the unit parts fit?

Thanks,

Cash

Click to expand...

If your head and barrel arent 9 stud then its worth while trying to find the 9 stud parts, as problems with head gaskets very common with the 8 stud ones.

hi cash,i agree with carbonfiber go for 9 stud head and barrels,but the problem with 8 stud was,nt gaskets it was cracking around the valve seats,

I've got a nine stud Bonnie head that's totally crack free, and the cases are big bearing alternator 6T. I've found a couple of rough unit motors at a knock down price and was hoping to salvage all the bits I'll need from them. But, what'll fit ?

Cash.

What wont ?

maybe left foot shift bearings are differant dia. but a few thou can be machined of to fit the early bearings.

72 crank with square counterweights on flywheel is 72% balance , but differant frames like differant %adges, maybe.

Just use a large oil pump. Had one on a T100 of a 750 someone (Livingston) reversed to fit .
Maybe the oil pump holes are a over t .reversed .

Setting up the cams and followers deserves a bit of care ,Needle roller bearings , same as box , can be used .

73 750 Valves CAN be used in 8 stud heads. Theres a few castings , maybe the available ones ARE the replacement for the Crack prone ones.

70 heads biggest ports .in a T120.

69 Commando / atlas points housing and Electronic ign .? Though a good Mags fine. pre 56 they had a smaller output shaft, which will shear .

see if you can find a set of W&S valve sprins . set for Yam XS 500 & 750 4 valve gets you all sorts of fancy bits, seats , caps , shims. They fit Commandos too . . .