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Discussion Starter · #1 ·
I'm looking at a 70 frame and would like to go with a 750 since i'm a bigger guy (6'1 250) and wouldn't mind the extra torque to get my butt movin'. My question is will a 750 bolt in to a stock 650 frame? Secondly is there anything more to a 750 motor other than the top end? I know these are newbie questions but I've been putting this endevor aside for awhile and would like to get it rolling this winter.
 

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no problem to put a 750 ia 650 frame;it's the same motor overbored!!you can do the overbore yourself by fittng a morgo kit (or another make...)
in fact this solution is probably better as the 650 cams are a little hotter.you just need a good bottom end (and eventually a crank balancing with the oversized pistons.
ben
 

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hi,just fit a big bore 750 kit to the t120 motor.use the t120 cams as they are better than t140 cams.the con rods in a t140 motor are different to the t120.as for piston weight,the big bore pistons will be the same weight as the 650 pistons so no crank balancing is needed.the difference in power delivery is at the bottom end.pulling at 2000rpm.a t140 pulls at 5000 rpm.only weak point is the t120 duplex chain.the extra 100 cc stretches it so fit a good new one to start.my 750 kitted t120 works great.you will also want a 20 tooth gearbox sprocket to raise the gearing a little.go for 8.5 to 1 pistons.
 

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If you have a 750 eng on hand, by all means bolt it into that frame. It drops right in, but the oil lines are bigger on the 750, and the head steady is different. I think you will like the 5 speed trans of the 750 over the 4 speed. My brother and I did this same swap several years ago, when his T140 burned up the day before the annual British bike ride. I had a T120 with a blown eng, so with the help of an all night auto parts store,we put his engine in my chassis. He put about 250 mi. on it the next day. That was the same day my Commando got blown into the weeds by a Vincent Rapide.
(But that's another story.)
 
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Discussion Starter · #6 ·
Well its good to know that these motors are straight swaps. The motor is probably going to be the last bit in my puzzle so I'll either part one together or keep my eyes open for a deal. Being in the burbs of Chicago I've also been told to check out the swap meet out at the McHenry county fairgrounds. There's a local shop called Morrie's from Richmond, IL that usually has a spot there. My roommates dad has a pretty nice inventory of British, Italian and 50's Harley factory racebikes. He's not to thrilled about my plans for a bobber so he let me know I should not bother with a matching number's frame and engine combo if its something I'm just going to ride. I'll have to dig up some pictures of his stable.
 

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The 750 engine is about 3/8" shorter in overall height, so it's easier to put a 750 engine into a 650 than the other way around. The stroke is the same at 82mm, but the 750s use 6" long con rods, center to center, and the 650s uses 6.5" rods. Shortening the rod with the same stroke moves the max torque point down the rpm range. The factory fitted a larger lift/longer duration inlet cam (Thruxton profile) for more low/midrange torque, and then installed an anemic exhaust cam. If you have the engine apart, get a 1969 or later 650 exhaust cam (.314" lift)....earlier ones will work, but are not nitride hardened. I usually take one from a 71 or 72, which are pretty common. Keep the 750 inlet cam, which uses the STD tappets...this combo works great with open/low restriction exhaust systems. (If you have a 650 hot rod, this works great in them, too). With this set up, you can use the low compression (8.5 or 7.9 to one, depending on year), but I like the MAP 9.5 forged pistons. I would consider rebalancing with any aftermarket pistons, or if you have overbored more than .020".
This is a little more complicated, but if the bike is light and the gearing not too high, you can take a light flywheel from a 66-68 650 and replace the heavy flywheel on the 750 crank, then rebalance the whole assy. This makes for a snappy engine, but not for heavy bikes or for mostly two up riding. For a barhopper/bobber solo/streetfighter it makes an interesting combo.
I like a light valve train (tappets, pushrods, rocker arms, adjusters, nuts, spring retainers, valves) for less wear and tear and snappier performance.
The later (mid 73-on) uses the oval port oil pumps, which are the highest output standard pumps readily available and will retrofit into all unit engines.
The 750s also use heavy cam pinions, which act as flywheels to smooth out the cyclic pulses of the cam lobes induce in the valve train. I would use these in either 650 or 750s and I believe you will have better valve train life. The piston type oil pump also causes a pulsing, as it is driven by the inlet cam pinion nut.
I have found more instances of clutch problems and broken gearbox mainshafts in the 750s, moreso than with even hotrodded, 750 kitted 650s which make more horsepower and torque. I suspect the triplex primary chain moves the center of effort outboard just enough to affect the loading on the clutch assy. and mainshaft. Easy fix, if you have a triplex sprocket and clutch....just use a duplex 650 chain fitted to the inner two rows of teeth. This also lightens up the primary drive.
If you have access to a lathe, you can make four top bolts for the rocker boxes that are like the 750 bolts from the rocker box surface down, and like the 650 from the surface up....you need a stud or a threaded hole in the 3/8" thicker bolthead. I would make it with the stud, as it's stronger and you are less likely to strip threads. This will greatly facilitate the installation and allow you to keep the stock top stays.
 

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hm mechanicca,looks like a design for my 1971.t120r .mine has the big bore 750 with the t140 oil pump and the 8.5 .1 pistons with a duplex chain and 20 tooth final drive sprocket.runs dead smooth.best engine i have built.vibration not a problem.took me 30 years to arrive at this combination and it works.
 
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