Center cylinder lost all compression - your guess?

I've been running Maxima full synthetic 2 stroke oil.
Not anymore though after 2 burn downs since switching oil. I sucked out all of it and will go back to my old proven semi synthetic brand from now on.

I have not been running on ice, good snow conditions.

Thanks for thr water temp gauge tip. I've actually been thinking of getting myself an exhaust temp meter. Wouldn't that be more relevant to spot these lean burn downs?
 
Hello,

I've got me a flywheel puller, micro meter (for pistons) and some feeler gauges (for checking piston/cylinder clearence and crank tolerances). The crank case is now split. Only problem I had was stripping all allen bolts to the flywheel but sorted it out in the end.

Haven't had time to inspect anything yet but the seals looks ok to me after quick visual inspection, will change them anyway. I did however find some aluminum pieces here and there so I don't regret opening the case at all.

Question is, how to tell if the bearings are ok, just sound and smooth feel?
Should the side bearings next to seals sit tight or not? Mine just slipped off the shaft easily.

Some pics:

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well from when my buddy showed me on a 78 et340, you want the bearings all to roll smoothly with no hitch or noises.
 
Ok, thanks.

I meassured the pistons last night and the new SPI were pretty much within tolerance at 68.92-68.93mm so they should be ok to use with a set of new rings.
The old OEM pistons that I replaced last time were at 68.85mm. They still showed good compression figures at 120/130psi while these new ones were only at 90 psi.
So I guess the rings are really worn on the new ones.

The nicasil is gone by a good thumb print both above and below exhaust port on the bad cylinder. Nothing to be done except another re-plating for around $200.
New piston set of 3 is around $200 while just rings with 1 new piston go for around $100 so I guess the right path is to go all new.

Any good advice on cleaning crankshaft bearings? I was thinking of spraying a can of WD40 while rotating crank and not be tempted to use high preassure air...
 
Hi,

So Im cleaning part by part an hour here and there. Tonight the carbs were taken apart and into the all new ultrasonic cleaner.

When Im putting it back together I feel a bit uncertain how the plastic washer on the main jet was supposed to be. Does this looks right to you? I tried a few other ways but this seemed to be the only way bottom carb case could fit.

Screenshot_20190623-232221_Gallery.jpg
 
I just noticed a damage to the crankcase. One of the stopper ring grooves is broken.

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Anyone seen this before?
Is the crankcase junk now?
Will the stopper ring rotate in that groove or is locked in the installation position, if it rotates I guess it could potentially cause major issues...what do you think?

Just as I am waiting for all parts to come in to start the re-build...
 
Boy, if it were me, and I had it that far apart already, I would for sure get a different lower case half.
Should be able to find one pretty cheap on Ebay.
 
The groove is partially gone on both the upper and lower case so going that route I'll probably get a set anyway.

So you think risk of blowing it all to pieces is considerably higher using it?
Atleast the groove on the crankshaft bearing is intact, perhaps some kind of bearing fixation paste to lock the stopper ring in position where the groove is intact could help it a bit...

I know the safe route would be a new case, problem is money vs chance of 2nd rebuild success and parts like that are not easy to come by cheap here in Sweden.
I'm used to order parts from US already but with import and shipping charges price will atleast double from ebay prices.

Example from ebay just now:
Subtotal (1 item) US $149.99
Shipping US $95.28
Import charges US $58.37
Order total US $303.64
 
I've been lucky and found an entire engine with a crank bearing failure for a reasonable price.

I will check it out in the upcoming days. Atleast I'll have some spare parts then since it's an all or nothing deal.

About reusing base and head gaskets, have you done it and do you use spray like Loctite 3020 or similar? My new base and head gaskets from 1st rebuild still feels new and only got a couple miles on them. Ive got new for crank seal, intake, water pump and power valves. Exhaust gaskets from Winderosa did not survive the teardown so thinking of re using original gaskets there as well.

I've ordered a leak down tester for monitoring ring wear but perhaps it can be used before first start to check if there are gasket leaks also.
 
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I picked up the other engine this weekend and as far as I can tell it looks good.
All three cylinders were in good shape and pistons looked used ofcourse but no failed ones.
All other parts were also in decent shape so I've got lots of spare part now.

When I compared the two cases I started to see that the old case had more damage to it. Strange that I didn't notice it before.
It's clear that the crank has been moving and scraped wear marks on the case in one direction.

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I'm speculating here but the wear is on the opposite side from the ring groove which could indicate its a direct result of the engine running with the ring alignment out of place.
I guess this could very well be reason for the burndown since all that friction must have created a lot of heat, and debris.
Do you guys think that the reason for these marks are the broken ring groove that made the crank become unstable or do you think the reason for both the marks and the broken ring groove is a bad crank?
 
Update: Today I took the crank to a professional mechanic. He will true it and check overall condition. Sent in both cranks so he can replace outer bearings if in better condition.
Also got the ProX pistons and Threebond 1184 so it's getting close to rebuild time.
 
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