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So my buddy has had this AHMRA spec lower end for a CB350 hanging around he scooped up off Ebay. Nicely done electric start chop/weld, bump start only, and beefy "Suzuki" Rods with a wrist pin opening slightly bigger than stock. We have been trying to figure out what kind of pistons may have been used with this case. I haven't measured the rods yet, but they are the same stroke, beefier, and have a ~2mm larger wrist pin opening. Anyone have any ideas on what pistons to use? Its a later model case, 4XX serial with single gear clutch basket.
Check out WISECO's catalog for Suzuki 2 valve pistons - maybe a GS400/450/500 or an early 4 cylinder. Be open to the possibility that the bore may have been larger that 67.5mm which is the largest legally allowable in AHRMA's 350 cc classes.
I'm not saying it was larger, just being open to the chance that it may have been.
Just found out the Studs are much larger too, a 17mm bolt fits in it, need to find engine studs too I guess.
Here are some pictures, seems they took the outside bearings off, drilled out what I think is a pin holding together, then rewelded it back together. I'll have to get a picture of the weld on the crank later. There is also an added runner for the bearing on the generator side I have never seen before, I do not have a picture of it though.
Stock Pin
We've got a stock crank in during this picture, here is the electric start block, the outer case is modified nicely for it.
pretty sure the 350 never had a single gear, could be modified for racing by removing one of the gears. looks like a bronze insert in the conrod, nice if that's the case, I'd probably have it pressed out and a new one machined to match your current wristpin, maybe even have a spare set made... seems like you got a well modified bottom end, good score
agreed with sean. interesting block off there... i like it.
im still curious why the modified bottom end... i havent heard of bottom end failing, even when lightly lightened with a motor pushing close to 50 horses.
Stock, they had two gears on the crank and clutch basket. Standard race trick is to remove one and save some weight.
That crank looks like it has stock heavy wheels. The bottom end may have been machined to a different stroke and the replacement pins welded in. So perhaps it was a short stroke (or longer), bigger bore than stock.
Any help identifying the rods will be greatly appreciated. Its quite a nice piece, the shift forks look brand new. The CB350G (Disc Brake) is the one with the larger single gear clutch basket. The crank definitely has not been lightened, both weights are identical to original.
More pictures.
Clutch Side Weld
Generator side weld, with notched bearing (For magneto?)
70mm is biggest piston you can get into a CB350, (unless you offset the bores, then you can go to about 72mm)
I know you can't get a 74mm piston into a CB350 as I've tried (and failed )
It's a lot of work plus cases need boring.
Possibly use CB450 pistons?
Thank you for identifying them PJ. Do you think something huge was in it judging by the 17mm Engine Studs? Any idea where we could find some studs that diameter but proper length? So far we've planned on using Henning pistons, good valve springs, new valve seats, and a Type 1 cam. Also have a head we did with a 35mm ported intake.
Excuse the painted upper end, just for mockup, I think my partner has a little ricer left in him :
I just took a look at those crank pics and I'm not seeing and signs of welding or machining on the inner wheels. That suggests that maybe the stock big end pins may have been used - or maybe we just need to see from a slightly different angle.
391cc cheating? In racing tell me it ain't so. For years we raced a legal 175cc Honda and some of the competition were using 200cc or more - before that was legal and it was obvious at times. You'd think that a 391 motor would come off corners much harder than most others but maybe it wasn't as well developed.
PJ - are rods available for the 350 or was that perhaps all that the builder had access to?
Got the XS650 studs and a M10-1.25 Tap. They are just slightly longer than CB350 studs, but definitely usable by all means. Took all but a half hour to have the holes retapped and the studs able to thread. Now whats left is to bore out the holes in the Cylinder, Head, and Cover for them and oil to run through. Then the part I feel will be the biggest pain, making larger set pins, and fitting them to keep it all from jostling. Should be one strong engine.
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