Interesting discovery on my SR-100

[Mike Anderson]

Started porting the new right side case for my engine and realized the studs in the racer cases are 7mm versus 6mm bolts in the 100 case. After thinking about that for a while and some more study noticed that the cylinder would not fit the case either. That being said it amazed me that they actually made special cases for such a small run of bikes as the sr-100's were. The stock case half could be modified with some more grinding as polishing to allow the cylinder to fit in but it might weaken the case. Drilling the stud threads and tapping them to 7mm will also cause a weak area. That particular spot on the stock 100 case is actually drilled thru do to less material in the area. All and all that area of the cases are lighter on the stock 100 than on the racer. Fortunately I think the original if fixable with some careful welding and machining, if not I have a spare set of SR-100 cases to use. I will post some pictures of the case differences just for other members reference. Merry Christmas and Happy New Year.
Later Mike

[slawsonb]

Interesting for sure Mike. Bridgestone was apparently very serious about their racer models and spared no expense, as the use of special cases (and alot of other things) would demonstrate. Keep the info coming, as an aspiring racer owner, I find this stuff very educational. thanks...
...bert

[paul]

Were the SR90 "special case" engines?

[Mike Anderson]

Back to the issue with the right side case on my sr100 engine. Purchased a set of Taka 100 cc cases to use for donor parts. Have the right side case cleaned up and ready to open up the porting. Will match up to my original and send back to Steve Reed for ultrasonic cleaning and reassembly. Hopefully the original case can also be saved but I have some doubts. The area where the casting is broken out is very thin, I think welding will be questionable. Have another idea to try machining a separate piece and attaching from the back side. Wish me luck. I am attaching some pictures of the Taka case compared to the original.
Thanks Mike

[slawsonb]

Was just thinking, I know 350 and 175 cases were machined as a pair to get the crank alignment correct. There has been discussion not to assemble cases with mismatched numbers (not pairs). Is there any similar concern with cases for the singles? Were they machined as pairs as well to square the crank to the cylinder mounting surfaces? or is it common practice to interchange cases on singles? Just rubbing my worry beads.
...bert

[OldSwartout]

A check of the 90-100 parts book indicates the crankcase halves were available individually, so there should be no problem interchanging parts from different engines of the same series.  I know some other brands of singles had crankcases machined as a pair, but apparently not Bridgestone.

[Mike Anderson]

Hi Karl,
Thanks for clearing that up for everyone. I knew they were sold separately but I will probably use the matched side from the Taka anyway. Nothing special about that side on the racer so no reason to mix parts. Both halves will have the same finish that way, would like the motor to look like it belongs together. Will post some more pictures soon, I have done the port work on the right side Taka case, everything looks good and should work nicely.
Later Mike

[disc_valve]

Hi Mike,

The matched case problem only occurs with horizontally split cases. In manufacture, the raw cases are first machined across the horizontal joint face and then clamped together as a pair to machine the bearing housings and outside joint  faces (transmission case and carb housing). The snag is that each pair of cases will sit in a slightly different position whilst the housings and joint faces are machined. Using an "odd" set of cases means that the bearing housing will probably not be truly circular and the transmission and carb housing faces will not be truly level where they cross the crankcase joint.

With a vertically split engine, the problem doesn't occur because none of the bearing housings or face joint run across the crankcase split. So you should be able to use either right side case on you SR. The only advantage of using the matched "Taka" set is that they are likely to have weathered the same so will be the same colour.

Incidentally, on the subject of stud size, didn't the 100cc road engines use biigger cylinder than the 90 anyway? My TMX I think uses the same head nuts as the 350, which would make the long through studs 8mm. The 90cc engines used short 6mm studs just holding the cylinder flange.

Graham

[Mike Anderson]

You are exactly right, 90 and 100 cases are totally different. Started porting on a new 90 half which didn't have studs installed by mistake. Fortunately the Taka cases were available. Would like to use the original case but repair is pretty risky. Hate to have engine back and running then have failure in weak weld. As I said earlier, plan is to try a different repair approach just for the experience. A machined boss will be made and the broken spot on case machined off. Then the boss can be bolted from back side. Still risky but nice experiment. At the end of the day getting the engine together with the proper porting is the main goal. Had hoped to keep all the engine parts original to the bike since restoration is not my goal on this one, but I don't think it is a good idea. Onward and upward.
Thanks Mike