The Engine - Bearings

Main Bearings

The LH case main bearing, which has to take the load of the offset flywheel, is a 35mm x 80mm x 21mm single row, cylindrical roller bearing. Look for SKF N307.

The RH side main bearing is a 35mm x 80mm x 21mm deep groove single row, unsealed ball bearing. Look for SKF 6307.

The LH flywheel side bearing has next to it a metal plate and a circular felt oil seal that sits in its cavity within the bearing housing. The felt oil seals were from Stucchi but can be cut from a felt sheet. I have used a circular saw but you end up with ragged edges. Jerry Kimberlin very generously agreed to make a batch of felt seals from me using a special cutter he made. He says that "it is not too hard to make a cutter. They are aluminum and on a mandrel that I put in the mill (or drill press if you have one, I don't). The cutting edges are concentric and as deep as needed for the felt used. In use, the cutter is put in the mill and spun against the felt which is on top of some wood for backing. Felt is easy to cut and is similar to other soft stuff like rubber, teflon, gasket material, etc. The difference between this cutter and a hole saw is that the cutter doesn't have any teeth but is dead smooth and sharp. Also the cutter has an inner and outer cutting edge so the felt ring seal is cut in one pass." Thus spake the oracle!

The RH side main bearing just sits in its cavity without any plate or felt seal. All the bearings except for the special size gearbox clutch and output bearing were from MSC Industrial Supply.

The clutch bearing in the LH case is a 30mm x 62mm x 13mm and the gearbox output bearing on the RH is a 33mm x 66mm x 13mm. Both these bearings were from Peters-Bearing in Germany. The two smaller gearbox bearings in the LH and RH cases are identical and are standard 15mm x 35mm x 11mm SKF 6202 deep groove, single row, unsealed ball bearings from MSC Industrial Supply.

The photo of the LH flywheel case shows the N307 roller bearing, plate and oil seal next to the cavity in the bearing housing and the oil seal on top of the incorrect clutch bearing. The second photo of the LH case shows all the bearings pressed in with the new original spec clutch bearing. The photo of the RH case shows the 6307 main bearing, the smaller 6202 gearbox bearing at the bottom and the special size gearbox output bearing. The metal disk in between the rear gearbox bearing and the felt oil seal has a step which should be facing the outside and be adjacent to the oil seal. I have reversed it in the photo to show the step.

The RH case in the blurred photo shows the special gearbox bearing with the four screw holes welded up and the bearings installed.

To check if the main bearings are correctly seated, push and pull the flywheel after it is mounted on the crankshaft. A radial movement between 0.03 to 0.05mm is tolerable but should not exceed 0.10mm - you should feel no or nearly imperceptible in and out movement of the flywheel.

Some quick notes on the 3 felt oil seals and the two metal plates. The felts are just as big in diameter as the cavity in which they fit. The ID of the main bearing felt is the same as the OD of the crankshaft that goes into the LH main bearing. The felt to seal the RH gearbox bearing has an OD equal to the diameter of the bearing housing but the ID equals the diameter of the spacer that fits on the gearbox shaft and the kickstarter. The metal disk for the LH main bearing is 1.5mm thick and has the same OD as the cavity for the felt seal and ID equal to the crankshaft. The second metal disk is for the RH gearbox bearing and is 1.5mm thick with the OD equal to the OD of the bearing while the ID is the same as the OD of the spacer that fits behind the sprocket. This disk, which goes in between the bearing and the oil seal has a 0.75mm deep recess cut into it on the RH side facing the seal with an OD equal to the size of the felt seal.

There are two brass bushes in the gearbox on both the RH and LH crankcase. On the RH side case, the upper bush is for the toothed gear selector shaft and has an ID of 19mm+-0.02mm with max wear clearance of + 0.04mm. The lower bush is for the gear selector drum and has an ID of 14mm+-0.02mm with max wear clearance of + 0.03mm. (The Super Alce, which was designed for military service mostly in the the Africa campaign, has 0.01mm more max clearance than the dimensions I have given which are for the GTV). On the LH side crankcase the upper bush for the toothed gear selector shaft has an ID of 15mm+-0.027mm with max wear clearance of + 0.04mm. The lower bush for the selector drum has the identical ID to the RH side of 14mm+-0.02mm with max wear clearance of + 0.03mm.

Next, installing the bearings. Since the key is to get uniform heat rather than a local hot spot that can cause distortion put the cases in an oven. For alloy 100 deg C (212 deg F) is a good temperature to aim for (to install any valve guide into a cast iron head use 150 deg C or 300 deg F). Classic Bike quoted former AMC metallurgist Don Hewitt advising that the melting point for typical British motorcycle alloy is 645 deg C (1,193 deg F) and 1,200 deg C (2,192 deg F) for typical British cast iron. If you don't have one of those fancy point-and-shoot thermometers the best way to gauge when the case is the right temperature is to look to see if Ms. Manners is watching and then spit on the case. If your spit sizzles then it is the right temperature. The previous day I had put the bearings in a zip lock bag and tossed it into the fridge. I took them out and put Loctite 620 bearing retainer on the outer ring. Then I gingerly lifted the hot cases out of the oven and put them on the workbench and with my heat gun reheated the bearing boss till it sizzled. I then dropped the bearing right in. With the exception of the LH side roller main bearing they all went in smoothly. The roller bearing (the inner race separates from the outer ring and stays on the crankshaft while the outer race is held by the crankcase bearing housing) needed a light tap (obviously on the outer races) with my bearing drivers to settle in. The tops of the races were all flush with the sides of the cases. If the RH gearbox output bearing needs to be tapped in do it gently as there is not a lot of metal behind and the entire housing could detach.