650 Piston Dimensions

Hello Everyone,

I am working on a 650 engine (18-97876) that I acquired partially assembled. Once the cylinders were installed the engine would not turn over and seems to have 2 different interference positions. at BDC the pistons touch the flywheel, and mid stroke I feel a hard stop when the rods hit the side of the cylinder.
The cylinder casting # is 18773, it only has a slight relief for the rods, not a full notch like a commando.
Also the flywheel has no reliefs in the center section and a H stamped at the top center.

It looks like I will be trimming the bottom of the pistons, and relieving the cylinders. Does anyone have insight on piston dimensions ( roughly how much I need to remove) and why the cylinder is interfering with the rods? Is the cylinder from a older short stroke model? Same with pistons? (only marked ID3 on top)
I have a commando cylinder to measure the notch cut out, thinking I will copy that.
Thanks in advance everyone, this is a great resource.

Hope this helps...
(and they are flat topped with valve pockets - not domed)

Your barrels look like 99/600cc not 650. My 650 barrels have full rod cutouts like atlas.
Does your crank flywheel have cut out relief for piston clearance when at BDC ? Yours appear to be atlas not 650. can't see enough to tell.
The fin pattern may also show it to be 600. There are pix somewhere that can show the evolution of barrels.

It seems quite common for GPM pistons to touch the flywheel, my 650SS had marks on the skirt where it had been touching when it was pulled down after a minor seizure, also an engineer friend of mine found the same problem on a 650SS he was asked to finish assembling.

here's some images and dimensions from a freshly opened stock 650 motor:



The dimension of the bottom of the pin bore to the bottom of the flywheel relief cutout on the piston.



Peering into the case at bdc. You can see the flywheel has a relief to accommodate the piston skirt. The relief does not go all the way around the flywheel, but only toward bdc, where the skirt would foul. The relief is about .150" at bdc. It appears you have an Atlas or Commando flywheel.









The 99 cylinders should work fine with the rod relief. Don't know what cam you'll be running, but you should also check that the push rod tunnels aren't too narrow with a higher lift cam.

Good luck.

Thanks everyone, good advice on checking cam clearance, time to start carving. The barrels have a boss for the generator strap/ mount. (99/600 probobly?) With the atlas or commando crank I wonder if 650 pistons would have cleared.

KHK said:

Thanks everyone, good advice on checking cam clearance, time to start carving. The barrels have a boss for the generator strap/ mount. (99/600 probobly?) With the atlas or commando crank I wonder if 650 pistons would have cleared.

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It's not the cam clearance, it's the later fatter aluminum pushrods that will rub on the exhaust side of the tunnels with the higher lift cam. Steel, constant diameter pushrods might be ok.

As far as the flywheel goes, no. The (stock) 650 pistons would hit, or at least scrape badly. Might be worth looking around for a 650 flywheel. It'll bolt up to the cheeks of the Atlas or early Commando crank. If not for the clearance then for the balance. More heavily carved piston skirts for clearance would further exacerbate any balance issues.

And just to make sure that your 99 barrels have 9 fins, not the 8 fins of the 500cc 88? Those are the only two with the generator boss.

it is not advisable to use model 99 barrels on the 650 as the 650 barrel are set back to the rear in the bore more than the 600 model 99 and this set back to the rear of the bore is so the crank can swing with more ease so I would recommend you finding a set of 650 barrels and save you having you engine letting go on you

annajeannette said:

it is not advisable to use model 99 barrels on the 650 as the 650 barrel are set back to the rear in the bore more than the 600 model 99 and this set back to the rear of the bore is so the crank can swing with more ease so I would recommend you finding a set of 650 barrels and save you having you engine letting go on you

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Is that so AJD? I didn't think that offset (rearward) occurred until the 750 (Atlas) - had believed the bores of everything up to, and including, 650's had the bores centrally over the crankshaft centreline.