Top End Maintenance: Piston Rings War

[proper]

Hello,

 

The missus has a nice Sherco 290 build '04, which runs great. 

 

There was however one small problem, and that was mainly the missus herself. She's tiny.

So we started out pulling it apart, softened the rear spring, greased the link system, cut 15mm from the front fork bushes, poured in thin fork oil, replaced what needed replacement, the whole lot.

End result: nice springy bike!

 

There is however another thing. She can't get it to run. I've explained it's mainly technique, but I have to admit, the thing has high compression, or, just the way you look at it, a short kickstartlever.

So, we went about adding a few washer under the sparky. Better, but not good enough.

We retarded the timing. No noticeable improvement.

So we ordered two base gaskets to try lower the compression a bit further. 

Removed the cylinder, removed all carbon deposits (There was quite a bit, must have run pretty rich)

 

After placing the gaskets, I tried installing the cylinder again, but could not get it to fit over the piston rings.

I tried to compress the piston rings with my fingers, but failed.

Then I tried using a large tie-rip. This would not compress the rings enough.

Then I purchased an universal piston ring mounting kit.

This kit features an open ended band, fitting around the piston, which can be compressed using a ratcheting set of pliers.

 

After two failed attempts to get the piston in the cylinder, I tried to compress the rings a bit more.

After that attempt, I was left with two broken piston rings.

 

So, to put it short, it's going smoothly so far.

 

I have ordered new piston rings, but before I start another attempt: I desperately need some advice how to get this simple job done!

The bike can be taken to the workshop anytime, but to me, this should be an easy job, to be carried out by me several times a year. I'm willing to master it.

 

Vincent

 

Edited October 23, 2016 by proper

[mcman56]

Page 27 shows one method.

 

http://www.lewisportusa.com/manuals/sherc_11-21-09/sherco_top_end_maintenance_manual.pdf

[lineaway]

 You do see the locating pins and the rings have to fit the ends at the pins.

[dadof2]

I can generally manage by holding the rings in place with mu fingers in the transfer slots in the "liner". Another method is to tie the rings tightly in place with fine copper wire Then seal the rings into the grooves about 30mm either side of the locating peg with candle wax melted well into the ring and groove. Remove the wire, scape off the excess wax. Once the barrel is fastened down heat the top of the piston to allow the rings to spring out. The rest of the wax soon burns off.

 

If you are fitting new rings always put them into the barrel and check the end gap first.

[axulsuv]

Never needed anything but my finger tips ... on a two stroke that is . I think Lineaway hit the crux of the problem ....!

Edited October 23, 2016 by axulsuv

[tony27]

Fitting the rings is definitely a fingers to compress only job, the ends of the rings will be offset from the centreline by about the same amount but opposite directions which hopefully is visible in this shot of an old piston 

As a point of reference the inside of the piston is marked Beta, I know this is the exact same piston used in both the Beta 270 & Sherco 290 with Vertex changing the insert in the die for the different manufacturers

 

[goudrons]

I too think Lineaway might be on to it.

The ring grooves on the piston have a peg in each groove, this fits the gaps in the rings.

These pegs are usually offset by about a 3rd but miss the transfer ports in the liner.

 

Often after a rebore/recoat, with a new piston and rings it can be a bit tight and fiddly to fit the barrel over the piston with the piston fitted to the conrod as it flops around, so it might be worth trying to fit the piston to the barrel first, like this:

 

Fit the rings to the piston (correctly with the pegs and the ring gaps lining up).

Fit one circlip to one end of the wrist pin hole and paint the piston with 2 stroke oil.

With the barrel loose on the bench, turn it upside down and align the piston square to the liner and start to feed it in. (the right away around, usually the piston crown is marked, this mark goes to the front/exhaust)

The liners bottom edge should be chamfered so helps to squeeze the rings as it goes in, use your finger nails to help squeeze as you push.

Don't twist, just push.

 

Once the rings are in, the bottom of the piston should still be hanging out with enough clearance for the wrist pin hole.

Fit the base gasket and stuff clean rags or paper in the crankcase mouth to hold the conrod vertical.

Fit the small end bearing to the con rod and lower the piston/barrel over it and fit the wirst pin (from the side without the circlip in).

Carefully fit the second circlip and remove the paper/rags

Now carefully push the barrel down.

Loosely tighten the barrel down and slowly turn the engine over with the flywheel to see if it all spins correctly before tightening it all up.

 

It's worth lightly greasing the base gasket or both if you are using two.

This often saves them sticking and ripping apart if you need to strip is down again later.

 

Edited October 24, 2016 by goudrons

[proper]

Thanks for all the great replies!

 

@Lineaway: Locating pegs it is. I never knew these existed. In hindsight, there were enough clues, but I just didn't pick them up. This is just so obvious, but I guess I'm never to old to learn.

 

At least now I'm confident the bike will soon be up and running again. Again, thanks for the great support.

[naichuff]

Just an add on Carbon can also build up at the bottom of the groove not letting the ring down in its groove (new rings are bigger than worn ones)  I always clean them out using an old broken ring be careful not to remove any metal of the piston

Edited October 24, 2016 by naichuff