What happened to my Zen? Attn, Engine guys.

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Trip

Trip

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Ypsilanti Michigan U.S.
Zen g260 stock in my FG MT. It ran great, but I was convinced by others I needed to put on a 28.5 bb kit, boy was I wrong!

The bb kit was simple enough to install, but it never ran correctly. One day it just wouldn't start so I dicked around with it and gave up. I let it sit for a few days and I pulled the air cleaner off and found some scratches on the piston. So I yanked the head and the piston was FUBAR.

Swapped back to the stock head and it was better for about half a tank, then it too started acting up. I again pulled the air cleaner and I saw some scratches on the piston. Pissed off, I put the motor on a shelf for 3 years...

Tonight I decided to pull it apart and here is what I found...

The reason the motor didn't want to run was because I had the intake gasket installed incorrectly and had the pulse hole covered by the gasket. Otherwise it probably would have kept running, at least for a little while. The compression is still great. Better than my CY GP290 has ever been.

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Whatever it was that wiped out the bb head was still in the case when I put the stock head back on, because these pics are identical to the pics I took 3 years ago of the bb head. I never cracked the case apart..

This is an old pic of the bb 28.5 head from 3 years ago:
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Back to the 26:
The exhaust side looks ok really, the scratches are superficial.
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The circlips are still in the piston, incorrectly installed I see, but nonetheless they are in there.

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Is this common for this gasket to be hanging out like this? I've never taken a case apart.
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Any ideas what would have caused this? I will tear the case apart tomorrow to see if there's anything in there. Has anybody seen anything like this before?

Thanks guys,
Trip..
 
Trip,turn the crank around and look at the bearings(big-end) That looks exactly the same as when I popped me head on my 'T' .The big-end bearing failed and lobbed the remains into the head.
J
 
well the first thought is a circlip came out. but if both are there, its unlikely, UNLESS>>> you inadvertanly installed 2 circlips on one side and didnt realize it. the spare circlip popped out in the 28.5 and was in the bottom of the crank during the re-install of the 26 head.
other option is a piece of the spark plug came off, or a piece of the bearings, or you just sucked something thru the carb at some point when you were working on the air filter. maybe a screw came out of the throtle or choke plate in the carb. ect....
 
almost looks like the little pin that holds the ring popped out possibly.

But whatever it was, it started on the intake...something sucked in and as it bound up the intake port, it pushes the piston tight against the cylinder transfer boss which did the damage in the cylinder transfer port side.

Any damage on top of the piston? Thats sometimes can mean that a bearing came apart, allows excess legnth on the rod and the piston contacts the top of the cylinder or bottoms out on the crank.
 
When you swapped the dead 28BB kit for your good original 26cc barrel and piston did you split the crank case and replace the bearings & seals?

If not then I would be my first born son on the fact that the damage was caused by the bits of the 28cc kit hiding in and around the bearings and seals. I've see in hundreds of time on customers 2 stroke machines at work and on peoples rc cars.
 
OK Let me address all the questions...

It does look like needle bearing problems to me to, when I tear it apart I will inspect the crank needle bearings.

No damage to the top of the piston.

Circlips were not both installed on one side of the piston, they are both still installed on the piston. When I tore off the 28.5 head 3 years ago both circlips were still on the piston.

I am assuming whatever took out the 28.5 head was left in the case and took out the second head.

The Piston ring-stop is still in the piston.

Spark plug is ok with no damage, nor is there damage to anywhere in the top of the head, so whatever it was is either still in the case, or was blown out the exhaust.

Look at the 3rd picture from the top (the blue pic) see the dents on the top of the exhaust port? They are round and relatively large, after reading Voo and Markster's posts, I think it may have been a needle bearing. I don't think a circlip would have caused that "diameter" of a dent in the ex port...

All great ideas guys.

I have to pull the flywheel off, other than using bolts threaded into the flywheel to pop the it off, is there any other way to remove it?
 
I have to pull the flywheel off, other than using bolts threaded into the flywheel to pop the it off, is there any other way to remove it?
__________________

Yes there is,my way of doing it is a sharp but gentle tap with a hammer to the magnet side of the flywheel.
J
 
the bolt method works best for me, a puller is a better idea but ill assume you don't have one, so go with the bolt method.

do you run that motor with an x-can? ive read that they can cause issues with sucking debris into a motor. haven't seen any pics, but it is possible. id also say a bearing coming apart, seems to be the most logical. add in the running problems due to not having the crankcase pulse (intake gasket in wrong) and there could be a little bit of lean sieze starting.
 
OK Got it apart, it was easy, I just put a 13mm socket and 3 inch extension on the flywheel nut and popped it with a hammer. Popped right off. I didn't even get a chance to read the suggestions... Thanks Guys.

Got the case halves apart and I found the little bastard that caused it.

Here is how I found it.
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It's .027" (.685mm) in diameter...
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Here it is compared to a rose joint.
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I don't see any other damages, nothing wrong with the needle bearings or the crank bearings, seals look OK too. Although I see the crank is all shined up where it rides in the bearings, it looks like it's been spinning inside the bearing? Is this normal? It's not from the seals, they sit out on the shaft a little farther out.
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Now I gotta ask, where the hell did this piece of steel come from?
 
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Here it is... It was a circlip... I just measured another one it's .027" just like the piece I dug out of my motor.

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Well at least now I know!

Thanks for all the help guys!
 
Sherlock strikes again !!
Sorry for your misfortune and the thought is possibly scary for all of us - could happen anytime ...
My G260RC engine in the MT is still very strong, but is starting to approach middle age, so it's one of many possible things that can go wrong I suppose.

No sign of anything as yet - wheelies away, and with that top-end Dom 'spike', the nose of a HEAVY heavy (its 18kg of alloy of various descriptions) it picks its nose up no problem !!!
No other major mods.

Al.
 
That looks like a needle bearing to me, you need to check the connecting rod for side play. You do this anyway anytime you do a rebuild. If it's out of spec you have to replace the needle bearings.
 
This motor stock with my JetPro was wild! Lots of fun... But the GP290 I bought is a pig of a motor, my 23 has more ass.

Got it all cleaned and put back together to sit on the shelf until my ported head arrives... Then it will be put in the buggy with a J/P Marder pipe, should be wild! The head is being "Radically Ported" by TMR Performance strictly for use with the J/P pipe. I should have an RPM in the high 20's! My 290 won't rev past 15k :lol: Stupid pile of monkey snot.

My chrome 290 is staying in my truck and it's going to sit on it's shelf until I get the desire to fix it... At over 25 gallons on 290 and over 15 gallons on the stock 260 motor, the chassis and drivetrain parts are getting sloppy.
 
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markster said:
That looks like a needle bearing to me, you need to check the connecting rod for side play. You do this anyway anytime you do a rebuild. If it's out of spec you have to replace the needle bearings.
Click to expand...

Check side play on the big end? It's sloppy as hell... I would say it moves at least .050"-.075" I just put it all back together for now. When the head gets here it will be coming with a crank that's not the "Typical" R/C crank. It will be knife edged and a few other tricks. I'll post up pics when I get it..

As far as that not being a circlip, I don't know for sure if it is or not. I thought I removed 2 circlips 3 years ago when I swapped the piston back, but it's been so long I can't remember???

I checked the bearings in the big end and they are all in there. I didn't check the small end because I didn't really care. I just mic'd a roller bearing from the small end I had in my pitbox and it's .058" (1.47mm) Guess it wasn't a bearing.

I guess with the tight clearance between the piston skirt and the cylinder it must have straightened out the clip?
 
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Do you guys realize there were 18 new inductees in the "Introduce Yourself" thread in the past 24 hours??? Damn!

Welcome aboard Guys and Gal.
 
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