Stroking the Virago

Viragos were built with very little spare room. The closeness of the heads limits carb selection and makes fuel injection difficult in the extreme. Equal length pipes can't be made without imperiling the rider's leg. The tightness of the crank chamber, the closeness of the windage tray and transmission gears, and the shortness of the rods make for a real adventure in achieving a longer stroke. To make something that would instill confidence in me would take a lot of inert gas welding and laser or water jet cutting. The bastard size bearings are very robust, but I wish that they were 6308s so that we could run spherical rollers with both an internal and external oil slot so that we could split off some of the transmission oiling to the mains. I'd over-drive the pump by juggling sprocket sizes and let the pressure relief keep the rods and top end from drowning.

So - kits will be guaranteed to install and start, but will only be covered for the parts you receive from us. Cranks, rods, bearings, piston sets, cylinders and I can't think of anything else at the moment. If you venture past redline and bend valves (damage that they do to our pistons is not covered.): You'll be SOL. Miss a shift and those potato chip thin cases flex and break, consult a welder. You get the idea. You have been warned.

Viragos were released with a set of very odd size main bearings: 40mmx92x23mm. I believe that is a 60309, but different suppliers mark them differently. They almost never fail, but they are so tight both on the crank and in the cases that the home mechanic tends to destroy them when attempting to remove them for inspection. It takes acetone in dry ice and a MAPP gas torch and abundent patience to extract them unharmed. They are available only in the C3 configration. The only sources are Yamaha at about seventy dollars per, and some bearing supply houses in Eastern Europe for not a lot less. They want to sell in quantity. A fairly common bearing is the 6308 at 40mmx90mmx23mm - the same, just two millimeter smaller on the OD. These are available in every imagineable configuration: I'm fond of double barrel rollers with oiling slots inside and out. For a lot less than the odd-ball bearings. Crank kits will come with new mains. I first determined that the ductile iron bearing bore in the case can be hard chrome plated down to 90mm. This would entail a case exchange program: too expensive and complicated. I'm told that some Nascar engines run bearing bore ID reducers and this is not uncommon orgnology. I see us installing some variant of the 6308 bearing on either end of the crank and providing some drive-in or press-in adapters for the customer to install before installing the crank to the cases.

Early inspection shows us that 108mm might represent too big of a bore, and 89mm too long of a stroke. The spindle that the cam gear/sprocket rotates on appears to be lubricated by oil scraped off the cylinder walls by the rings on the down stroke. A very large and/or long sleeve would close off the bore that houses this piece. It appears that a hole would need to be bored through the side of the lower sleeve and champfered to oil this critical part. This is not a unique idea. It would necessarily make each cylinder either a front or rear - unless a superfluous hole were put on the other side as well. And because the pistons pull partway out of the bottom of the sleeve at BDC, needed longer sleeves for strokers would come closer to slimmed down flyweights and this oil hole would be essential.

It appears that the low pressure transmission oil feed comes quite close to the pivot bore. A narrow, carefullly-executed drilling may do the trick. This disturbs me on a couple of counts: I have to trust people who's ability and attention to detail I cannot assess. And I need to develop some rather involved instructions. More as this develops. And a hold on this bore and this stroke.

I assembled a crank with rods and pistons to a case today to see what would work. A 106mm bore would work with an 84mm stroke. Maybe. I sent my design thoughts to Falicon to see if they thought that they would work. I run it backward and forward in my head and turn the crank to check clearances. The only machine work necessary would be the reaming of the case spigots to acompany the larger sleeves. The sleeves would need to come lower down and closer to the flyweights. Stock the pistons pull half way out of the sleeves at BDC. This causes piston slap and scoring on the thrust faces. We want to reduce and possibly eliminate this. At BDC the skirts would barely clear the flyweights. One of the front rod nuts comes very close to the back of the windage tray. Some filing may be necessary. Then again, we may get around this with the rod style we utilize. I am dithering between aluminum and a style where the bolts thread directly into the cap. No nuts.

How about 108mm bore by 89mm stroke? Sleeves large enough to guide these pistons would need to come very close together. Especially with a longer stroke. The flyweights will need to be reduced in diameter. Tungsten (or other heavy metal) plugs will be needed to restore balance. The windage tray will need to be dressed with a rotary grinder. Very carefully as a major oil passage moves from one case half to the other in the bottom.

Stroker kits will only be offered in 106mm by 84mm; balanced and complete. If I decide that it is possible, the same will be true of the 108mm by 89mm kits.

For some further analysis, see here.