Triumphrat

jimmy bush · 04-23-2009, 05:22 PM

I have seen several references made to the short rod 750 engine. I am curious as to the details about this. Were all the 750's short rods? I looked in my "Kim the CD man" Triumph CD and all the 750's from 73 to 79 use the same rod. The pistons were changed some where in that time frame but the rods stayed the same. The reason I'm digging info is I want to build up a street tracke of sorts with some spare parts I have. I have the cases of a '72'-'73' T140V. I have a 750 crank that I'm pretty sure came with the cases. I have some used 650 rods that are in pretty good shape. I have a 10 bolt cylinder and head from what I think is a '79' or later T140. The carb manifolds point straight back and are screwed into the head and the carbs mount with hoses, not bolted on. The cylinder is 1/4" shorter from head to base than the 650 cylinders I have. I have a Morgo 800cc cylinder and it is the same height as the 650's and only has 9 bolts. I have a '71' 9 bolt head that is in good shape. Is the combutsion chamber the same size on the 650 and 750 heads? I have a light 650 crank that I can take the weight off to swap with the heavy 750 weight if that would be a benefit on a street tracker. Anybody know what will work with what? I don't want to get down the path and find out half the stuff can't be used with the other half and have to back up to take a differen't route.

gearhead · 04-23-2009, 10:03 PM

The T140 (750) engines used a shorter rod then the 650 engines, the stroke was the same the pin was relocated in the piston so the 750 rods must be used with 750 pistons and the same for the 650 parts, the cranks should interchange i don't know about the balance other than the factory recommends 85% balance factor for the unit engines. Don't know about the combustion chamber size but i would use the splayed port head its supposed to flow better. Sounds like a fun project.

Webby · 05-22-2009, 05:45 PM

Sounds Fun,

The crankshafts should be interchangeable between the 650 & 750, but I think you would need to use the original primary drive due to the duplex chain on the 650 & the triplex fitted to the 750 (the crank may be slightly longer, anyone out there who could confirm

As for the rest I figure it should fit.

Good luck

Webby

mark61120 · 05-23-2009, 09:06 AM

The '74 and '75 650's used triplex primary chains.

Mark

Webby · 05-25-2009, 05:59 AM

Thanks Mark,

You learn something new every day