500 seizing on a fresh top end.

[gday]

I just rebored my 84 500, fitting a wossner 87.42 piston and bored to wossner spec 0.08 mm clearance . After a gentle 15 minutes 1&2 nd gear trail on a cool day it locked up on a moderate incline straight at about 1/3 throttle. On tear down there was significant seize at 10 o'clock to the exhaust port at 12 and a lesser seize at 4 o'clock . Oil mix was 40:1 iPone full synthetic . Prior to the rebore the bike had run without issue under heavy load to the same fuel/ jetting / timing spec. The rebore was carried out by a reputable shop . My check indicates its all to spec. I've never had an issue with seizing or nipping up in 40 years of riding ( other than an 83 kdx200 on a trailing throttle which were too tight from the factory ). I've heard the "run them in oil rich " theories , but I don't consider 40/1 synthetic lean.
Has anyone had any issues like this ?
Clearly I will need to replace he piston but without knowing what the cause was I concerned it will happen again.
Is the wossner spec too tight ?
Do the wossner pistons grow unevenly?

Glenn

 

[Paul P]

.08mm ( .002 ) is way to tight for clearance. Pistons that are about 75mm and larger will need about .15mm, or, 3.5 thousanths, give or take. I would hone out the clearance to the 3.5 Th, and still break it in the way you were doing.

 

[Bigbill]

I'm not sure? Things are missing from your story.

1. Was this a running motor you wanted to freshin?
2. Did you do a leak down test prior to disassembly?
To check the crank bearings and seals?
3. You didn't touch the bottom end?

I'm not sure what you did work wise.

I rebuild the complete engine, crank bearings, seals, gaskets, piston. I coat the piston with moly by burnishing it into the piston skirt and cylinder walls. Then add extra moly on the skirt. Then I per lube the seals and crank bearings with two stroke oil. During assembly I add two stroke oil to the wrist pin and cylinder wall.

Liquid or air cooled the piston to cylinder wall clearance is different

I also check the piston for being out of round. Just incase.

I always check the location of the ring gap pin that holds the piston ring from turning. I did find a problem with an experimental cylinder and piston I tested from the husqvarna engineers for there 100cc chainsaws. My saw was the only one that completed the test.

I seen guys refreshin the top end after a seizure and not address the crank seals and crankbearings. I also do a leak down test after the engine is assembled too.

 

[Up-tite]

I think Bigbill meant check cylinder for being out of round.
Pistons are out of round and tapered and have a cam grind to them.

All pistons grow unevenly, look at the piston on the underside 10 and 4 oclock is where the piston has the most material when heated this is where it expands the most 12 exhaust means too lean.

Don't confuse oil/gas ratio to jetting as far as lean or rich.
This is one thing that confuses people a lot. If you are jetted properly running 60/1 then go to 20/1 you could have a problem, because you have in reality reduced the size of the main jet buy the fact more oil going thru the jets and reducing the amount of gasoline.

It's the gasoline that keeps the piston cooled down not the oil. Need some oil for lubrication, piston,rings,bearings. But gasoline is the cooling agent. Put some gas on your hand then wave it lot cooler?

Get it cleaned up new piston then do a leak down check.
If ok go up on jetting pilot 1-2 sizes lift needle 1-2 clip positions Main 1-2 sizes.

DO NOT ALLOW YOU BIKE TO IDLE. Set the idle so if you start to walk away you get 2 steps then it dies.
Once broken in and only if riding in tight conditions can up idle but if lots of Hi -speed no idle you will seize it.

Break-in 2 full tanks of gas ride it like you have a broken foot. (Take it easy) then re torque head.
Now it 's time to check jetting. If all good pin it.

Later George

 

[justintendo]

i have also seen where as a top end wears and loses compression it will require leaner jetting to make power...then a rebuild comes along and it seizes on previously dialed jetting..

 

[gday]

Thanks for the good feedback , I will consider all the points you've raised as I work through this problem.
By the way :
This was a top end freshen up on a engine that was running ok.
I hadn't done a leak down test ( it wasn't showing any signs if crank seal leaks - not to say it isn't leaking though!) so this might be in order. ( how hard is it to do the crank seals on a husky?)
The oil ratio and jetting was what I'd always run - no change there.
The piston clearance is as per wossner spec ( which nay be too tight- ref attach photo if the wossner spec )

I will do some tests and more measurements and let you know.
Thanks again.

Glenn

 

[Bigbill]

I believe when we change the bore diameter we need to rejet it. Please correct me if I'm wrong.
I admit it I bored a 390 once too tight too. But a quick pass with a gear adjustable sunnen hone fixed it. I was lucky.

 

[gday]

I don't know. I wouldn't have thought the tiny increase in bore for 1 over would have a significant impact on jetting but I will consider everything . It didn't feel lean , if anything it felt a but rich. Thanks.

 

[husqyhamm]

Questions:
What was your ring gap measured at piston tdc position.
Jet sizes?
main, air correction, idle, atomizer tube no, needle no and clip position?

 

[stormer254]

Billions and billions of engines have been rebored with no changes to the jetting, if all other things are okay it can really only be the clearance.

 

[gday]

Jetting is a big richer than stock
40 mikuni
380 main
55 pilot
R07 air jet
Needle jet AA-0
2 slide
7DH3 needle middle clip

 

[gday]

That should read "bit richer "!( damn these iPhones and fat figures) I didn't measure the ring end gap prior and it's stuck in the piston now !

 

[fran...k.]

I've never had an issue with seizing or nipping up in 40 years of riding Has anyone had any issues like this ?
Clearly I will need to replace he piston but without knowing what the cause was I concerned it will happen again.
Is the wossner spec too tight ?
Do the wossner pistons grow unevenly?

Glenn

I have had this issue with Iron air cooled four stroke cylinders and fresh piston done to spec or supposely. If there is a way to check for out or round that should be done.

Is the wossner spec too tight ? same numbering on top to make spec plus number come out to a whole or a half mm just like mahle origional.

Do the wossner pistons grow unevenly? I think so they are much like a wiseco in that the insides are machined out and a more massive front to back structure at the wristpin boss area.

 

[adam6402]

The conversion of .08mm is not .002, it is. .003. I recall somewhere on here that the forged Wossner's clearance gap should be .004 on the 430 and 500's as too many problems at .003. Someone mentioned that desert racers would go with a .006 clearance to avoid seizure after long WOT runs.

 

[juicypips]

.006 opens up piston slap an orrid noises running, thats the upper wear limit on a knackerd piston isnt it?
Either way yup it sucks when things dont go to plan.
Wossner know what theyre doing as a manufacturer and i would trust their tolerances above a guestamate.
My manual states 30:1 running in mix then 50:1 after heat cycles etc.
I know gas is the coolent but the oil is the slippy and if there is not enough that can cause picking up on pistons etc, how shagged is the piston could you dress it gentally as wossner arnt cheap.

Good luck with sorting it.

 

[Paul P]

.08 on any metric conversion comes to 2 thousanths. Way too tight for an 87.5mm piston.
Wossner pistons are very similar to Wiseco, and Wiseco pistons will expand 3-4 thousanths in the first few heat cycles. If you want to test this, try an experiment, measure the piston cold, fresh out of the box, then put it in a pre-heated oven at about 200 degrees for a minute, then measure it when hot. You'll be surprised at how much it grows.
In my methanol/nitro 2 strokes, I needed to run 8-12 thousanths clearance with the Wiseco pistons before I could get the bike to perform well. Most race companies that do special motors start at 6 thousanths for a 500cc motor. But these motors run at race speeds, not in trails or tight tracks.

 

[adam6402]

I must be a math idiot, how do you arrive at .002 from .08 mm, 1mm=.0393 inches X .08mm= .003?

 

[Paul P]

Sorry, I stand corrected, .08 comes to .003. Not hitting the right buttons with my glasses off.
But that is still on the tight side for the bore with a Wossner piston. Without being there to run the bike, there could be a lot of other problems. Even not rinsing the cylinder properly after the bore can ruin a good bore, and piston, even bearings with little run time.

 

[DaveM]

Listen to what George said, leak down test! your motor might have a air leak, check rubber inlet manifold for cracks.

The only time I had a exhaust port seizure similar to yours that was when my bikes fuel tap became blocked at 1/3 to half throttle causing the same looking type of seizure.

I reread your post and I see you had a multi point seizure?

I would like to see a pic of all sides of the piston.

Listen to George, tight cylinder clearances, easy riding for a least 1 - 2 tanks of fuel, no lugging bike up steep hills.

Research 2 stroke seizures, there are sites with pics so you can identify from the piston what was the cause of seizure that you had.

http://www.goforwardmotion.com/pdf/troubleshooting.

Cheers, DaveM.

 

[gday]

As requested - Here are a few happy snaps of the piston after I tried without luck ( obviously) to revive it in the feild with a wire wheel in an atempt to get going for the two day vintage enduro last weekend .
Thanks for the advice fellas .
Glenn