More finds from inside the SF1 engine

O

olly

Senior member
Plastic teeth pretty chewed up - new set of those right?
Screenshot 2021-02-10 at 11.44.07.png

Uh oh...compression ring has been broken for a while - bore looks ok
Screenshot 2021-02-10 at 11.44.18.png

The other ring looks like it's been waggling up and down and opened up the groove, the bulged out top of bottom of groove have worn. Pictures of each piston for comparison.
Screenshot 2021-02-10 at 11.44.38.pngScreenshot 2021-02-10 at 11.44.47.png

New piston too?
 
Looks like new pistons. Why did the ring break?
Measure it all up.
New tensioner wheel obviously but they all become like that pretty quickly.

Paul
 
yup - the bottom end almost certainly is the source of the KNOCK/DONK noise - valve, cam etc all fine at first glance...I'll keep this thread ticking over as we dig deeper.
 
am I right in thinking that those plastic wheals start of as a plain round item?
been along time with delving into a 750, and no thanks I don't want someones engine to refresh my memory with.
CLEM
 
Yes they start as smooth round wheels with two raised sections to go between the runs of the duplex cam chain. They end up looking toothed after not very long, but I have found that the SFC items with needle rollers in them last far better.
Olly, have you checked the primary drive sprocket on the crankshaft left hand end yet?
 
My top rings were in pieces after the motor had done 60,000 miles, the last 20,000 of that with no air filter (and riding dirt roads with genuine bulldust in northern Australia!). The rings broke because the ridge at the top of the bores was so big! I had to go straight to 2nd OS.

Ring lands wear on these pistons - I'd be going a rebore and new slugs and rings for sure on that motor. You would never fit new pistons to old bores.
 
Tippie said:
Yes they start as smooth round wheels with two raised sections to go between the runs of the duplex cam chain. They end up looking toothed after not very long, but I have found that the SFC items with needle rollers in them last far better.
Olly, have you checked the primary drive sprocket on the crankshaft left hand end yet?
Click to expand...
Ummm...so they're going to wear like that anyway - where are all the chips and bits going? Now they've worn nicely surely the speed of wear is now at it's lowest and it's best to leave alone?

I'm not with the bike - but I assume he's checked the sprocket. We've got to take it all apart anyway because base gasket leaking and metal bits in the oil.
 
Whilst 2 new oversize pistons and re-bore is certainly best. If you are stuck and need one replacement standard piston, I have one + rings. Approx 30,000km. Happy to provide photos etc. This was after I melted the other piston. Good one all looks OK, not obviously worn, ring gaps within tolerance etc. Send me a PM if interested. I'm in Wales.
 
If you stick a new ring in a worn piston land, chances are that it'll break quite quickly I imagine.
Have a look at the crank before placing an order for pistons.

Paul
 
olly said:
Ummm...so they're going to wear like that anyway - where are all the chips and bits going? Now they've worn nicely surely the speed of wear is now at it's lowest and it's best to leave alone?

I'm not with the bike - but I assume he's checked the sprocket. We've got to take it all apart anyway because base gasket leaking and metal bits in the oil.
Click to expand...
The nylon wheels get brittle with age and heat cycles, they can break. Cheap stuff, replace them. It appears from what you have shown from the insides that you are up for a full rebuild.
 
Now you know where the noise comes from.
The rh main bearing isn't usually the main suspect, it's the left.
Don't worry about the bearing seat in the cases.

Paul
 
Paul Marx said:
If you stick a new ring in a worn piston land, chances are that it'll break quite quickly I imagine.
Have a look at the crank before placing an order for pistons.

Paul
Click to expand...
That is off the cards. The slot is worn. New ring will waggle up and down more than it should
 
Paul Marx said:
Now you know where the noise comes from.
The rh main bearing isn't usually the main suspect, it's the left.
Don't worry about the bearing seat in the cases.

Paul
Click to expand...
Will replace with roller bearing. Is some sort of loctite or equivalent needed?
 
RobinSF1 said:
Whilst 2 new oversize pistons and re-bore is certainly best. If you are stuck and need one replacement standard piston, I have one + rings. Approx 30,000km. Happy to provide photos etc. This was after I melted the other piston. Good one all looks OK, not obviously worn, ring gaps within tolerance etc. Send me a PM if interested. I'm in Wales.
Click to expand...
@RobinSF1 have sent you a message
 
A large amount of the plastic chips are now in the crank oil slingers... crank would need to come apart anyway, judging by overall condition. RH bearing doesn't generally give up without reason.

The top rings break up when the groove is worn enough for the ring to flutter. Bearing bosses are nothing to overly worry about, use bearing/shaft adhesive when assembling. Gearbox will be just as knackered...

piet
 
Definitely a motor in need of an overdue rebuild, thinketh I. Never seen a RH main fail on an SF motor. In fact, never seen any main bearings fail on an SF - I'm sure they do, but generally they're safe as houses. I'd hazard a guess the cam and primary chains are well past use by date as well. Done some miles, that donk!
 
BTW, I did a rebuild on an SF1 motor and discovered that the drain feed hole from the R front head stud down to the RH main had never been drilled! Luigi must have been distracted or rushed off for lunch and the arvo siesta :p
 
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