Size Limits

Without welding or casting, the available bore size is limited by the distance between the nearest across-the-bore studs. This appears to be 117mm. For safety's sake, we want to leave at least 2.5mm of aluminum outboard of them. If the stud thread were coarser I'd seek at least 3mm, but it is so fine that it represents a lot of surfaces which must be sheared to cause failure. This reduces the circle to 112mm. For safety/reliability, a minimum of 2mm (.080") sleeve thickness should be maintained. This leaves us with a max bore of 108mm. At this size, it becomes necessary to bore and champfer a hole to feed oil to the cam chain drive. This is usually fed by splash, but the wider sleeve masks the area that is normally fed. Thus, oil scraped off the bore on the down stroke will be pushed in.

A 112mm OD sleeve needs a lip 116mm wide and at least 2mm thick, It will fit with some notching for the stud passages. Shedding heat becomes a serious problem. Other larger displacement twins rely on long strokes (and low revs) and push rod tubes -- no cam chain tunnel to slow the dissipation of heat. Short stroke big twins are all water-cooled. An XV so bored would have to be used only on short runs, or use some sort of liquid cooling, Thermo-siphon may be the simplest. I have scoured used parts lists for a simple pump that could be mounted to the clutch cover, but I have found nothing appropriate.

Now then: stroke. On XV's (1100s included) there is about 6mm of clearance between (I believe) mainshaft second and also above the windage tray. This is as close as the nearest rod nut approaches. So, 81mm looks like no problem. A limiting factor is how close the rod journal comes to the top of the "crank chamber." If money and machine work were no object, One could run a smaller rod pin - even going to a pressed-together crank with roller bearings and one-piece rods. I see a limit (of piston-to-piston at BDC) of approximately 89mm. With a roller crank, one could: limit the pressure feeding the crank and send more oil to the top end. Hard chrome the main bearing bores down to 90mm and run double row roller bearings, with an oil feed split off from the transmission.

108mmX89mm yields 1631cc, which I see as the limit without a complete redesign. Some cooling system would be mandatory.

At least two years later:

Okay, so modifying the cases to accomodate smaller bearings is untenable. So is modifying the type of rod bearing and the basic rod structure. So we'll stay with those bastard-sized main bearings and two-piece rods with a fifty millimeter journal. I have never seen worn out or damaged rod bearings - except where someone put an oil filter in backwards.

I keep doing my checking with empty cases; I kept ftechetting how crowded forward the transmission is. 81mm may well be the stroke limit. Further checking with a transmission in place is called for.

About an hour later:

Well, the first transmission that I found was from an '84 Virago 700. Throughout the series the right crank drive gear and the clutch basket with its attached gear (as well as a ton of other stuff) all sported the same part numbers. Clutch hubs varied in the 1100s. Knowing this, we can conclude that the distance from the crank to the trans mainshaft is the same. And since the gearing was similar for all, the tooth count variations will be trivial. For the record, I am using a set of '82 chain-drive cases and an XVS1100 crank of unknown vintage with 920/1000/1100 rods - not the longer XVS rods.

There is far more room than there is for the rear rod's nuts to the back of the windage tray. I didn't bother to measure, but it looked like an easy 20mm. My previous estimate of 6mm was dead reckoning by looking through a case spigot at the trans. This was on an XVS1100 with its top-end off and rods still in place. The eye plays tricks.

This tells me that my previous assessment of 84mm is good with the same style rods and a slight massaging of the rear of the windage tray. Let me pause here for some clarification: a windage tray is usually a bolted-in construct of sheet metal below the flyweights of the crank. Its purpose is to block most of the oil collected below the crank from getting picked up and spun and splashed all over. When I write about the XV windage tray I am simply refering to the inner web where the case halfs are cemented together. There is no actual tray. Just a partially closed-off chamber to separate the spinning crank from the bulk of the sump oil.

As for the 89mm figure: there is no impediment "above" the crank at this stroke. The area below pretty much dictates a different rod configuration. My thoughts are for rods that have no lower nuts - the bolts thread directly into the material of the caps. One advantage is that the crank can be installed to one case half, the the rods could be installed. Or, if you're careful, the whole lower-end could be bolted up and then both rods could be mounted. This would certainly make getting the case halfs together much simpler.

To avoid piston slap and bore scuffing the sleeves will drop down deeper into the cases. For the longer stroke, this will require flyweights of smaller diameter, weighted with heavier metal plugs to maintain a reasonable balance. The shorter rods will spend more time motionless at TDC and BDC, then require greater acceleration and piston speed. This is good for low-end power. It allows for (one might say it requires) long cam timing and big carbs. It also limits maximum safe engine speed - or at least, effective speed. Where a stock stroke 750/920/1000/TR1 might safely spin to 1100RPM, the longer stroke motor should theoretically be okay to 8500, it will run out of breath at 7000. And even with longer sleeves, the upwardly accelerationg pistons will tend to "outrun" lubrication, And will scuff even when they don't. I am leaning toward ceramic/Nickasil/chrome on aluminum, but the pistons would not last that much longer. Still, it is cheaper to replace pistons than to restore monstrous bores.

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After playing with the pistons and cylinders, I see that at stock BDC they protrude just a hair. It's possible that kit pistons may have less skirt hanging down. Be that as it may, the sleeves must be longer to keep them (and compression) "in the loop." Stock, there is very little clearance between the flyweights and the sleeves; perhaps 15mm. If my thinking is clear and an 84mm stroke would have the piston 13.5mm lower at BDC, the flyweights must be skimmed slightly to maintain clearnace. And what of an 89mm stroke? The piston would go down 21mm lower than stock. The flyweights would necessarily need to be shaved much smaller and have heavy metal plugs inserted. Remember that the crankpin rises half the stroke increase higher. The long sleeves would need cut-aways 7mm higher than stock. My thoughts? The radically larger bores would be wide enough to allow the crankpin and rods to enter them whole, doing away with the need for cut-aways. I must make some measurements. It is possible that larger, longer sleeves would crowd each other to the point where their shapes must be altered.

These engines are tall for their displacements. This is due largely to the single cam square on top and its high mounted rockers. If they were DOHC with buckets over the valves or if there were pushrods instead the heads could be much squater.

Most motorcycle engines have rod ratios over 2:1 and when the pistons are at BDC they are located above the case deck. They are not near the crankshaft. The XV series, however, has the pistons below the barrels much of the time and they spend a fair amount of time close to the crank. Even though they have a respectable rod ratio, both rods being on the same throw makes for tight angles of attack and a fair amount of piston scuffing. A longer stroke and shorter rods exacerbate this. As they say, you've got to suffer if you want to be beautiful.