Base Gasket Squish

[clav3]

Done another engine rebuild and engine runs realy quiet.

As I increase the revs to about 1/3 throttle I hear an engine knock like piston hitting the head, not real bad but annoying.

So I've had a few lite hours of run in time.

I've taken the head off and inspected but no visable marks on the head or piston.

The only thing we didn't replace was the conrod assembly.

As I push down on top of the piston there is a slight tick noise like the bottom crank bearing needs replacing.

When the engine was rebuilt I have only ever replaced the base gasket with a standard one but I have just read on the splat shop web site that the standard base gasket is now 0.5 mm thick and they recommend a squish clearance of 1.2 mm and I may have to use two base gaskets to achieve the correct squish.

I have never checked the squish, just replaced with a standard base gasket.

We have to pull the engine apart again to fix this knocking sound but not sure whether base gaskets alone will fix the problem.

Any advice on Base gaskets or crank assembly replacment appreciated.

[dombush]

Clav

Apologies, this isn't a brief reply. If you replaced the base gasket that was already there (or one of a similar size) it's unlikely that the piston is touching the head (also, if it was touching you'd likely have destroyed the engine by now)

Firstly,was the engine knocking before you rebuilt the top end? It's unusual for engines to fail instantly unless starved of lubrication.

I would check the following in order of likelihood / ease of checking.

1. Check everything you removed / adjusted / touched to ensure something isn't vibrating / loose / hitting something else. (this is my hunch)
   
2. Check the plug to see if the motor has been running lean and detonating (also identified by hesitation when applying throttle sharply)
   
3. Check that the motor returns to tick-over quickly and holds steady revs (crank seals)
   
4. Check the crank bearing play on flywheel side by removing the flywheel side cover and try to move the flywheel up/down.
   
5. Remove the clutch cover and do the same with the crank pinion.
   
6. Check the squish (see below)
   
7. Check the big-end bearing play (see below)
   

If you've got to the "squish" on the list, this is checked as follows:-

• Select some soft solder approx 1.5mm dia and cut a straight piece around 6"-8" long (double up and twist to increase the diameter if necessary)
  
• Take the plug out
  
• Pre-bend the solder such that when threaded through the spark plug hole it will aim towards the cylinder wall (below the cyl head)
  
• Thread it through the plug hole and "feel" it touch on the cyl wall.
  
• Hold it there & get someone to bring the piston up to TDC using the kickstart
  
• Now apply steady increasing pressure with your foot onto the kick start (not a kick).
  
• When the solder has squashed (the motor will go past TDC) pull the solder out and measure the squashed end with a vernier or micrometer (the length of the squashed bit should be around 8-15mm long)
  
• If you haven't got a vernier / micrometer you'll have to use a steel rule and be very accurate.
  
• Bear in mind that the squish would have to be less than 0.3/4mm (125cc) and 0.6/7mm (290cc) to hit the cyl head at max revs.
  
• A good squish value depends on piston size but might be approx. 0.6-1.0mm (125) and 1.0-2.5mm (290). Engine builders change squish to achieve different performance characteristics or cope with different fuels. If in doubt go with the recommended value.
  

If you've got to the "big end" bearing on the list, this is checked as follows:-

• Remove the cyl head and barrel
  
• Get the motor to TDC (most big-ends wear at TDC)
  
• Grasp the con-rod/piston with both hands and push up/down with the con rod EXACTLY vertical.
  
• ANY discernible vertical play is bad (only vertical play counts)
  
• You may have to rotate the crank 90 degrees at a time and repeat to ensure the wear isn't elsewhere on the crank-pin.
  

Dom

[tltel]

Hi

If you look on the forums,there are instructions on how to check squish with soft solder, its eay to do. I had issues with my gas gas when I bought it and the knock does sound quite serious, although there were no signs of the piston hitting the head there was almost no clearance, i fitted two base gaskets and took the clearance up to 1.5 mm, that was about three years ago, and its still fine.

hope this helps.

just remembered, rather than buy the gg gaskets I bought several sheets of different thickness gasket paper and made my own, much cheaper and easier to get the right thickness.

TLTEL

Yes . What Dom Said. He posted while I was writing.

Edited April 1, 2013 by tltel

[copemech]

Clav, obvious first question, how many gaskets were there from factory? It seems to me that many just have the one standard gasket, yet some are adjusted slightly with the shim gasket added.

If you find no verticle play in the rod, it should be fine, and if it were going to knock, it should pretty much do it all the time I would think.

I will also mention the fact my "07 bike had a similar condition to what you describe from new. Made no difference what fuel used. Long story made short, I never got rid the noise untill I increased the pilot jet to a size 38 on the stock dellorto.

The standard range on base gaskets is 1 to 3 depending upon if you want to intentionally soften it by reducing compression and also increasing squish dimention. When I refreshed the top on that bike, it had the standard gasket plus a "shim" gasket of about 0.3mm best I could tell, so I just put two standards back, actually hoping to soften it a bit, yet I could tell no difference.

Edited April 2, 2013 by copemech

[clav3]

Only one Base gasket from new. The dealer here suggests only the standard which is what I've used before.

The engine rattle is definetely not fuel related!

It is engine rattle not pinging.

I must say I cannot remember the engine running this quiet before, which may accentuate the noise coming in at higher rpm.

Main bearing and seals are new, new piston and rings with correct tolerences.

Even checked the crank with dial gauge and blocks. The crank felt okay but wasn't checked until it was fully removed from the engine.

It's sought of like there is only slight wear but enough to cause a noise at higher rpm.

Thoughts on this theory?

Probably not a good idea to check squish if the bearing at the crank has slight movement.

I have taken the head off and rotate the flywheel until the is about 10 to 15 mm from top, push down sharply but gently and the piston drops slightly and makes a tick noise. Dropping slightly, I mean barely visible by eye.

It seems the conrod bearing should be replaced. (Conrod kit)

Edited April 2, 2013 by clav3

[copemech]

Obvious hard to say over the "tinternet" sounds are indeed subjective matter as well, yet if the bike has been well ridden it is not impossible.

These bikes do have a lot of mechanical noise to begin with, basic clearances, straight cut gears and such on the primary side, clutch and gear rattle.. Then add the fact you do a fresh top so those things are now quieted, one starts listning to other things.

Then one gets into the fact that normal evaluation of the big end bearing must normally be done strictly by "feel" , yet any percievable slack is not nood, and can be difficult to separate from lateral slide and rocking, as these things do indeed have some clearance which is basically hidden by the oil normally.

All said, if you feel it indeed a mechanical noise, best would be to be able to compare to a new on if possible, yet if you know it is just not right, replace the thing(not cheap) and be done with it! Yet as I re read your orig comment of (piston hitting head) what you dicsribe is a HEAVY knock, which would not be typical in a slightly loose rod condition that I might fathom, without it being more constant.

[tltel]

Sorry, but I find it a bit hard to believe you would do all that work on an engine and not check the con rod for wear. I think probably any engine I have taken the top off I have had a look to see if there is any movement, also that you didnt check the squish clearance just out of curiosity before removing the head again. I don,t think you said what engine it was, but am assuming its 290 sherco, and I think probably the squish is more important on the larger capacity engines,

Just my thoughts!!

TLTEL

[clav3]

Did check the con rod for wear and have done several times since using different methods and virtually impossible to feel any verticle movement at all. Thats why it was left alone especially at a cool $405 AU

Also checked by a guy who rebuilds engines of all types every day and is a long time trials rider.

The wear doesn't just happen over night, and I'm fairly sure it's at the start of being loose and thats why it's only making a noise above 1/3 throttle.

I can tell you that it's left us all scratching our heads. But the only thing left to replace is the bottom.

It is a 290 and since checked the squish. At the sides 35 to 36 thou which is about 0.9 mm.

Splat shop suggests 1.2 mm but the dealer here in aus suggests between 0.8 mm to 1.0 mm.

Anyway I received the parts today and there is a big difference in the side to side movement at the top of the rod.

So anyone reading this who wants to know what that might be, its no more than 0.25mm side to side if that, at the top with the bottom held in place.

My old one was about 1 mm more.

Neither feel any up and down.

I'll stick it in and let anyone with any interest know what happens.

Thanks for all the input.

[tltel]

Hi Clav3

just thinking on what you have said,I suppose its possible that in my case as the squish was very tight ( it almost cut through the solder I used) and there could be a small amount of movement on con rod, when the engine is hot and things have loosened up and expanded, the squish clearance would be vertually nothing and could cause the knock.It is a 95 engine so there must some wear. It sounds really good at the moment.

Let us know how yours turns out.

TLTEL

[clav3]

Have done a second rebuild... so, new main bearings and seals , piston and rings ( within suggested tolerances of 1.06 to 1.27 thou) with the variation in the bore of 0.2 to 0.3 thou, new conrod kit including all top and bottom end bearings, new clutch basket silent blocks rivets and plate, no sings of anything touching hitting or rubbing on inspection.

The engine is pristine. The cylinder could be re- coated to make it brand new again but I don't know if that will get rid of this persistent noise.

The noise is the same as it was before the second rebuild.

I have also added another 0.5 mm base gasket, squish is now 1.3 mm...made no difference.

I've put a timing light on it and the timing advances about 18 to 20 mm as it gets to about half throttle.

Does anyone know if this sound about right?

Just a note, the engine is running super smooth and is reving up normally, just has a horrible noise as it revs up.

I'm thinking of borrowing someones electrics to see if thats the problem.

I must say I'm getting a bit over it.

Not sure how to get a video up for anyone to listen.

Edited April 21, 2013 by clav3

[steve_earle]

Hi there, Have read your post and I'm guessing the noise is piston slap, If you have a worn bore or the piston you replaced is not up to spec it will happen. Get a proper 2 stroke engine builder to check it or stick the old piston back in to see how it runs, Might cost you a base gasket but will tell for sure if the noise didn't happen before the rebuild..

I've ordered pistons before, size a, b etc as stated on the old one and found the new one to be out and needed the bore matching to the piston.

The problem with bike manufactures is they use different companies, usualy the cheapest at the time like gas gas, They now only supply S3 pistons as they use S3 barrels and pistons now as standard. You can't get a standard piston unless you can source old stock. The S3 pistons in the standard barrel are not always spot on but this is only an example from my experience.

It's probably a piston problem, Ive recieved pistons from S3 and they don't even give proper size circlips for the gugeon pins.

[copemech]

Have done a second rebuild... so, new main bearings and seals , piston and rings ( within suggested tolerances of 1.06 to 1.27 thou) with the variation in the bore of 0.2 to 0.3 thou, new conrod kit including all top and bottom end bearings, new clutch basket silent blocks rivets and plate, no sings of anything touching hitting or rubbing on inspection.

The engine is pristine. The cylinder could be re- coated to make it brand new again but I don't know if that will get rid of this persistent noise.

The noise is the same as it was before the second rebuild.

I have also added another 0.5 mm base gasket, squish is now 1.3 mm...made no difference.

I've put a timing light on it and the timing advances about 18 to 20 mm as it gets to about half throttle.

Does anyone know if this sound about right?

Just a note, the engine is running super smooth and is reving up normally, just has a horrible noise as it revs up.

I'm thinking of borrowing someones electrics to see if thats the problem.

I must say I'm getting a bit over it.

Not sure how to get a video up for anyone to listen.

Right then, since you have gone through all this to no avail, entertain me!

So if I understand correctly, this is a "08 290 with a stock Dellorto PHBL 26? Is thayt correct?

What jets are in it? Hopefully around 35/ 120?

Put a 38-40 pilot jet in it and see what happens, you may have to turn the fuel screw in a couple turns.

[ham2]

Where are you going with this Copey? What d you think is causing it, as far as carburation is concerned.

[clav3]

2008 290 dellorto.

33 pilot 2.5 turns out and still reasonably rich down low.

I know you're probably sitting there thinking this guy doesn't know what he's talking about putting a 33 pilot in but three facts come to mind.

1. Bike has run sweet as for about 4 years with this carb setup.

2. Engine knock comes in above 1/3 throttle.

3. I have already tried lifting the needle to the max but no difference.

Forgive me Copey I know you,re trying to help but you need to get your head around the fact that not all 290 Shercos require the same jetting.

I've only owned two and my old one was set up much like yours but this one does not need a 38 or 35 pilot.

Today I spoke to a small engines mech that works on an ex Aus champs bikes and he had no doubt that the bike has a timing problem.

He had seen the exact thing before. That bike started fine, runs smooth down low but as you increase the revs, hey presto, engine knock.

On that bike, on the factory setting it dialed at 1.5 mm BTDC so he retarded the crap out of it with the same results, engine knock at higher revs. Put a new stator on and fixed.

He told me to dial gauge mine and 3.5 mm BTDC was the go.

I have another stator to try so we'll see.

Don't worry I'm not getting my hopes too high just yet.

[copemech]

2008 290 dellorto.

33 pilot 2.5 turns out and still reasonably rich down low.

I know you're probably sitting there thinking this guy doesn't know what he's talking about putting a 33 pilot in but three facts come to mind.

1. Bike has run sweet as for about 4 years with this carb setup.

2. Engine knock comes in above 1/3 throttle.

3. I have already tried lifting the needle to the max but no difference.

Forgive me Copey I know you,re trying to help but you need to get your head around the fact that not all 290 Shercos require the same jetting.

I've only owned two and my old one was set up much like yours but this one does not need a 38 or 35 pilot.

Today I spoke to a small engines mech that works on an ex Aus champs bikes and he had no doubt that the bike has a timing problem.

He had seen the exact thing before. That bike started fine, runs smooth down low but as you increase the revs, hey presto, engine knock.

On that bike, on the factory setting it dialed at 1.5 mm BTDC so he retarded the crap out of it with the same results, engine knock at higher revs. Put a new stator on and fixed.

He told me to dial gauge mine and 3.5 mm BTDC was the go.

I have another stator to try so we'll see.

Don't worry I'm not getting my hopes too high just yet.

Clav,

I appreciate what you are saying and I do not discount any of it. I have no magic numbers here as there are too many variables, seems things are not the same carb to carb, bike to bike, year to year, fuel to fuel, yet at the same time I learned on my "07.

In the range you are referring to, it sounded as though I was beating upon the head with a ball peen hammar!

I had never had that before, never ran more than a 35 jet and most were 33! A 36 made no change so I gave up for quite a while and just rode the bike.

Point of exercize reguardless of specific numbers, would be to find something to effect a change! Seems nothing you have tried thus far and at great expense has changed it. (save that conrod btw, someone will need it some day)

So all said, when thinking outside the box, I go for the easiest and cheapest things first!

Carry on and do report back,

Cheers,

MC