XJ900F engine/crank end covers

Hi

I'm new to this website so will have a few questions on the odd occasion. Hopefully I'm posting on the right forum with this query!

Having finished building a 1974 Triumph T150 I need something to do, so I have picked up a few XJ900F models, a 1993, 1990 and 1991.

I'm going to customise two of them, wire wheels, different fuel tank and seat, general re-styling etc.

One thing I'm working on is the round crank end covers. I'm thinking of getting some CNC machined with the Yamaha 'tuning fork' logo on them.

My question is why, on the XJ900F, are these two piece with a thick rubber gasket in between? On the XJ700 Maxim and I think XJ750 they have one piece covers, and yet aren't these engines pretty much the same as the XJ900F?

I can get the covers made as one piece or just the outer piece, but just curious as to why there is the difference.

Thanks for any help.

Shaun

 

Good question.
I have the single piece on my 900 from a 750.
I have a set of the 2 piece ones that are in bad nick but I also wondered at the time.
Len has to know!

 

I'm going to place a bet on the rubber pieces are there for vibration/sound dampening.

What do I win?

@chacal - any insight?

 

Hi

Thanks for the input. It's curious. The rubber gaskets/seals would suggest something to do with either vibration, although I can't think what from, or to keep something in or out, dirt, oil etc. If it was oil or dirt then you'd only need a one piece cover with a paper gasket presumably?

There is nothing under the right side cover that looks like it moves, but underneath the left cover is I guess the ignition advance rotor and electrical leads. I thought maybe the rubber gasket acted as a buffer for this, but then the gasket surface is covered by the two semi-circular steel plates which hold the outer cover on. So I really can't see what this rubber gasket achieves!

My only other 'wild shot' theory is that the rubber gasket, being so unusually thick (nearly 4mm), is there to protect the end on the engine/crankcases from damage. Any minor impact to either end of the engine covers would be absorbed by the rubber buffer, and if the end cover got damaged presumably it would be cheaper to just replace the outer part?

Would this make any sense?

 

Crash theory is about the best I can think of also.
Falling off the sidestand level of protection, mind you.

They do look cooler than the single piece ones though.

 

Well, that's all I can think it's for.

I'm working on a one piece design at the moment and I'm also looking at the large round insert that sits in the centre of the clutch cover to replace the standard plain one.

I like the style of clutch cover that was on my VMAX 1700.

Below is some indication of where my mind is wandering...

 

Do the covers with the YICS stickers have the same 4mm seal?

 

I have a 1983 XJ900, I think this has the YICS stickers on the covers. I’ll check to see if they have the 4mm seal. I have a feeling they do.

 

The YICS covers on my 1983 model are two piece.

 

Thought so cheers.