Benelli 250 SuperSport (egg)

[Lotus54]

I’ve had this Benelli 250 SuperSport (1973) for a few years. It only had 800 miles on it when I got it, but always had some shifting issues. Just recently I found out that the reason it was parked was a broken shift part- replaced. I tried quite a few things and never got it right, just better. So decided to pop the engine out and fix it. There is one bushing the siftshaft rides in that was damaged. I think perhaps someone didn’t get it seated right and then jammed the cover on or something. I was having trouble finding that one, but bought some oiled bronze and will make it.
I also though from the manual that the cases needed to be split to replace it. But turns out it can be tapped out without doing so. But I’m going to split it and make sure all the shift forks, drum and all the bearings are good. I will almost certainly just replace all the bearings.

The flywheel was a bugger to get off (as I mentioned in another thread). I tried all the tricks I knew, John suggested an air hammer- I didn’t have one so brought the engine out to his place. It didn’t come off with the air hammer, but after a LOT of heat and the factory puller- it finally slowly came off.

Next I needed to pull the primary drive gear. I saw a NOS factory puller cheap, so bought it. Got it today and it worked great. That gear was on there tight. I didn’t have anything for the cam gear, so I made a puller and it came off pretty easily.

[Lotus54]

Using a Tusk case splitter. Even though economy tool, it works pretty well and decently made.
Cases are apart and I’m pretty sure I found the issue. I will replace most if not all the bearings- just because.

[John H]

A serious looking tool. Do you use any heat to assist the separation? That's almost a 'must' on old British bikes.
Have you checked for run out on the crank yet? Seems it tightened up in the cases after trying to get the flywheel off.

[Lotus54]

I heated it up with a heat gun- but not that much. It came right apart.
Getting the crank out of the left side was a little harder. But I used that air hammer and the trick you had. Popped it right out.
I will be checking runnout and see if still in spec. I don’t like the way the bearings feel, so will replace.
There are four main bearings, with the outer ones being double row! I guess they didn’t want main bearing failure.

So extra thanks for the air hammer!

[Lotus54]

I have bearings all out and working on cleaning/vaporhoning and then ordering bearings.
The main bearings (4) are seated by circlips, so that means driving them out on the inner race. In any case, after getting them out and looking at them I am replacing them all anyway. I see signs of corrosion from sitting.

[Lotus54]

I have no idea why they use four large main bearings, with the outer ones being dual row. It seems overkill, but there must be a reason. All the bearings are SKF of the same type they still make and use. They can run pretty high rpm, they have a short stroke, although a pretty short con rod. I expect that does change loading.

[John H]

They sure put some quality in that little motor. Wonder why they didn't build a road racer with it.

[Lotus54]

They did make road racers with them- Zanzani did quite a bit. The factory did some- but mostly concentrated on the 250 and 350 fours that did very well.
This engine is developed from the Motobi engine. Benelli started with 6 brothers - at a point in the 50s one of them broke off from the others (can’t imagine 6 Italian brothers getting into arguments). He started MotoBi.
The early ones were 2-strokes, but this same basic engine came along pretty early. So perhaps with the oils and bearings of that time having 4 ball bearings for the main was required. But even later the bearings were bigger and better supported. This 250 ‘SuperSport’ engine has larger bearings than the ‘regular’ 250s and I see a different part number on the cam. Valves were the same size so I’m not sure how much different they really were.

The horizontal engine makes for great head cooling and low centre of gravity. With good frame and excellent suspension these little bikes handled fabulously. Still great fun.

https://www.pressreader.com/uk/the-clas ... 3833429792

[Lotus54]

Bearings in the cases.
Next I will fit the gearbox and verify no shim changes are required.
After that- time to install crankshaft, gearbox and finally put cases togehter.

[Lotus54]

I pulled this apart since it had shifting issues, especially downshifting from 5th down. I found out recently it was parked at 800 miles due to shifting issues- replacing broken shift drum ‘wheel’ (part with the pins that the shifter moves).
I found a bad bushing, nobody had a new one, so I made one from oiled bronze. From what I could tell, it appeared cases needed to be split to replace. After pulling the engine, I found it could be tapped out from rear of engine- but I already had it out and cylinder off etc. so pulled it all apart and replaced all bearings etc (only about 1500 miles total).

I put the cases back together with just gearbox (not crank etc) to test. It still didn’t shift right. It appeared the ‘dogs’ on the shifter was getting hung up on the shift wheel.
So I pulled out some spares both old and new. I can’t remember if I had replaced this or not- but this one has ’dogs’ too long!
I installed another NOS I had and it appears to work just fine.

Part that was in bike and not working right- another NOS part that seems fine.