12 replies to this topic

[gilbertdave]

Hi (again)

Ok, having rebuilt the bike after the last bout of issues (broken kickstart gear, sticking throttle, cracked airbox.... etc) I rode at a trial and was told by a few knowing souls that the top end has gone.

I'll not bother trying to explain the noise it makes, but it does definitely run with a rattle which has attracted remarks about the it being the new Gas Gas Diesel... blah blah blah. Anyway, I took it over to someone in the know and was told the noise is piston slap and that I should strip off the head and piston and send the head for nickosealing.

So, obedient as I am, I've stripped the head off this evening and I can't see anything wrong? Having been around when someone bravely dismantled their bike in the woods for the same problem, the head was passed about and everyone was making comments like "jeez, its mullered", "look at the state of that" etc. I never looked at it, but I assume it has to be pretty bad to be so obvious?

So, before I rush the parts down to my parts man what else should I be looking at?

The piston has side to side movement of about an inch, but it feels like the sort of movement its meant to have. Its slides rather than slops.

If I grab hold of the con rod and give it a wiggle there is teeny tiny bit of play as it disappears into the crank.

Finally, if it is decided i need all the new bits, how do i get the piston off? I can see some sort of clip, but can fathom how to remove it and I've learned my lesson with just bashing stuff now!

Any and all help would be great;ey appreciated

Dave

[serious3]

about an inch side to side in the bore?? 'kinell i'm surprised it started let alone ran!!

[gilbertdave]

Hmmm...not quite. I've taken the barrel off and the so now the piston is exposed and fixed to the con rod. The float is along the little end bearing I guess?

[lineaway]

Is it a rattle or a constant tick that changes with engine speed?

[gilbertdave]

its a rattle. It really does sound like a diesel.

The gas gas dealer who listened to it said piston slap and a few others have said little end. I have no idea

[MotoVintage]

you may just need a new piston, the cylinder may be fine, need to measure the piston and cylinder for clearance, remove the circlip to get the piston pin out, check your main bearings by grabing the flywheel and see if there is up and down movement.

[charliechitlins]

Flywheel should have a TINY bit of side-to-side movement.
No up and down.
The connecting rod can move side-to-side on the crank pin (an inch seems like a LOT though) but should have NO up-and-down movement.
The piston can have some scuffs on the thrust faces (front and rear), and the barrel can have some scuffs, too.
Knowing what your looking at is the only way to know if this is excessive.
If any material in the piston appears to be pulled or folded, it's definitely shot.
The wristpin clips may just be circular and you can easily pry them out if there's a notch in the piston. Look closely for a place to get a small pick or screwdriver in.
They may also be snaprings, and you'll need snapring pliers.
WHATEVER YOU DO, STUFF A RAG IN THE LOWER END when working with a piston.
You'll hate your life if anything drops down into your engine.

[gilbertdave]

The connecting rod has a very small amount of play side to side. The movement I'm talking about is the piston moving along the small end bearing. I'm fairly sure this is normal. I looked at a parts drawing and it shows the small end bearing is considerably wider than the small end of the con rod. I kind of assumed that there should be some packers or something to limit this movement, but the drawing confirms that what i have is normal.

In terms of signs of abuse, I really can't see any. There are no marks, scrapes or signs of deformed amterial inside the bore or on the piston.

Howver, if I grab the con rod in one hand and rock the piston side to side, there is slight play which makes me think its just the little end bearings?

Given that the little end bearings for my bike are less than £20, I'm inclined to replace it, rebuild the bike and see if theres an improvement? £20 and an hours work to rebuild seems like a risk worth taking if it means I save £350 for new pistons and nickosealing.

I'll also try and measure the gaps around the piston inside the bore. Can anyone tell me what to expect? I'm pressuming a set of feeler gauges for setting spark plugs should be ok?

Cheers,

Dave

Edited by gilbertdave, 02 September 2011 - 12:04 PM.

[B40RT]

Might be worth changing the wrist pin as well if they are not to expensive.
Also the retaining clips.

[charliechitlins]

I can't imagine how to get an accurate measurement of a cylinder without a bore gauge.
There may be a way, but I doubt it involves inserting the piston and using feeler gauges.
Feeler gauges are not meant to be used in curved surfaces.
And a piston is "cam ground"..... oval.
If you were REAL handy with geometry and math, I suppose you could insert a ring, measure the end gap, pull it out and measure the end gap again.
I suppose the right person with a calculator and knowledge of the relationship between circumference and diameter could get close.
If you can rock the piston like you say, your little end bearing might be shot.
Don't forget to check the big end for up-and-down.
If your new wristpin clips are wire (round cross section) they go in any old way.
If they're flat snaprings, they have a thrust side (it's flatter with sharp edges) and it has to face out.

[JSE]

charliechitlins, on 02 September 2011 - 02:09 PM, said:

I can't imagine how to get an accurate measurement of a cylinder without a bore gauge.
There may be a way, but I doubt it involves inserting the piston and using feeler gauges.
Feeler gauges are not meant to be used in curved surfaces.
And a piston is "cam ground"..... oval.
If you were REAL handy with geometry and math, I suppose you could insert a ring, measure the end gap, pull it out and measure the end gap again.
I suppose the right person with a calculator and knowledge of the relationship between circumference and diameter could get close.
If you can rock the piston like you say, your little end bearing might be shot.
Don't forget to check the big end for up-and-down.
If your new wristpin clips are wire (round cross section) they go in any old way.
If they're flat snaprings, they have a thrust side (it's flatter with sharp edges) and it has to face out.

You can try a snap gauge and caliper, but at the tolerances that the piston/cylinder clearance is held to (.0015/.002") it's not possible to be accurate to any large degree. You can get a relative estimate of ring wear (and extrapolate that to cylinder/piston wear) by measuring the ring end gap of the old rings (square ring in cylinder about 10mm down from deck and measure end gap). Knowing that the newly installed end gap is about .1mm per inch of cylinder bore, you can get an idea of how much the rings (and the cylinder/piston relatively speaking) have worn.

The piston kits come with a pin and new circlips. Be sure to check the cylinder for signs of excessive wear of the Nikasil lining. Usually about 10/15mm down from the top ("deck") of the cylinder on the exhaust side (known as the "thrust side" where maximum piston/cylinder pressure occurs during the combustion cycle) is a good place to start.

The best way to measure clearance is a dial bore indicator and micrometer. I'd also look at the airbox/filter/boot areas as there may be a problem with grit passing into the cylinder and you want to fix that before installing another piston.

Jon

[gilbertdave]

On the understanding that I dont easily have access to the technical equipment to make those assessments, I took it down the shop and let them have a look... I'm assured the wear is over the top and so its been sent off to be nikosil'd and I've got a new piston on the way.

I 100% can't afford that, but nevermind. I've also asked for gaskets, o rings and little end bearings. May as well?

In terms of the airbox issues (good memory by the way JSE), I feel fairly happy those are now resolved. I've put the bike through some fairly unpleasant conditions and there isn't any evidence of filth getting past the filter or any obscure openings in the rubber / plastic.

Before long I will have replaced almost everythying on the bike! I can lay claim to owning the newest old bike in the country!

[gilbertdave]

A little update...

The bike is now rebuild and sounding much better.

There was / is some minor complication which is causing teh bike to over run slightly when revved. a "man in the know" had a fiddle with the carb and it got a lot better. I'm planning to check out the jets at some point to see if I can't get it perfect.

But, the bottom line is, it is now mobile again and got me through the trial at Danbury on Sunday. Good times. I did however try to blame "running the engine in" for me fiving a section... truth is, I was just being a pussy with the throttle! ha ha