Shorrocks Outrigger Bearing

Peter le Gros, rider of the 8 second 500cc single 'Sledgehammer' has kindly supplied the following information, based on his experience of pushing a C75B to its limits.

THE outrigger bearing.

TO allow higher RPM and avoid drive shaft breakage, install an outrigger bearing. This will require setting the blower about 1.25" inches further inboard to give room for the bearing, which sits between the blower and the drive pulley. This is normally no problem as the Shorrock blower is fairly slim and would otherwise sit to one side of the bike centreline.
The bearing is simply installed within a top-hat shaped housing, which is in turn fixed to the blower drive-side end casing with a circle of c/s screws and nuts. The heads of the screws being on the inside of the blower, carefully countersunk to sit flush with the inner face of the end cover. I also put a smear of silicon sealant on the threads of the screws.

THE pulley is machined with a spigot whose o/d fits inside the bearing (see right). Incidentally, the standard drive shaft is a plain taper. If you can get your pulley to seat on this without slipping you are a better man than I. I always have the shaft and pulley keyed, but even then it still moves and rolls the key over. The shaft is soft steel and without the expense of manufacturing a complete new drive assembly there is little that can be done in my experience. Let me know if you have solved this one - I'd be pleased to know!

VANES

At high RPM there is a tendency for the blower vanes to try and part company with their carriers due to centrifugal force. Do not confuse internal scraping of the blower casing with this occurrence. This is perfectly natural and is caused partly by case shrinkage if using alcohol fuels, and partly by the very design of the blower.

Since the main supporting shaft is only fixed at one end (opposite to the drive) a certain amount of flex is inevitable at high RPM resulting in the outside edge of the vanes touching the inside of the casing, usually only around about half of the circumference. Incidentally, this will result in an excellent gas seal when operating at higher RPM, but of course gives

BACK to the vanes. Each one is fixed to its carrier by two rivets. Because of the design the spacing between each pair of carriers is different on each vane. Some vanes receive more of an uneven drive force because of this.
I simply run a thin braze joint down the edge of each carrier/vane joint. You could probably weld but be careful of the heat so as not to distort anything. TIG would be your best bet although I have not tried it.
Caution, some of the very old Shorrocks had brass carriers. I only ever came across one of these and used silver solder for the joint. This worked fine.
Be very careful to braze down the edges and not across the tops, or the uneven joint will interfere with the trunnions. They get closer to the vane carriers than you think. See diagram.

One important tip. Having extracted the vane assembly on your home-made mandrel, make up a jig to lock the vanes, bearings and all the little spacers firmly together before brazing (or welding). Something like a pair of straight edged clamps tightened across the outside edges of the vanes should do.
If you overlook this precaution the heat will allow the vanes to move on their bearings and the whole thing will end up out of line. When you reinstall everything you will then find one of the vanes scrapes on the inside of an end casing. I advise this from bitter experience!

ROTOR

The main rotor. The rotor is driven by the drive shaft and in turn drives the vanes around. It's the thing shaped like a washing machine drum and is located at the opposite end from the drive by a large bearing. Replace this with a new sealed unit, but while you have it all apart check the surface where the rotor sits inside the end casing. The o/d of the rotor is quite a tight tolerance with the i/d of the rebate in the casing. If these touch (flex, dirt or shrinkage) the rotor will lock up and shear. I always machine about 10 thou extra clearance out of the i/d of the casing here. Just a bit of insurance.