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Al
18-Jun-11, 01:38 PM
Where is the best place to buy spares for a 503 engine? Are there any pattern part available or only genuine Rotax parts?

I need a set of Pistons & rings, engine gasket & seal kit.

I lost compression on one cylinder, so stripped the engine down & what a mess:eek::(. Piston has started to break up:eek:. I need to get a rebore and a pair of oversize pistons. The good piston is 0.20 over size, I cant read the top of the other one but assume it will be the same.

I will also need a second hand head for the PTO side as this one now looks like someone has been there with a hammer & punch for an hour or two:(. Lets just say it is far from being as smooth as it should be:cry:~:((~:-((

Luckily the cast iron liner doesn't look too bad and should be ok with a rebore "dependent on how much over size I can get pistons.

I think what has happened is that a ring has probably been stuck, then broken up and a bit of it got on top of the piston and made a right mess.

As I have to split the engine to make sure there are no bits in the bottom I shall fit new crank seals & check big end bearings ect. There doesn't seam to be any up/down play in the pistons so I think that is a good sign.

Nick Drew
19-Jun-11, 12:08 AM
Hi mate,

You need to speak top Conrad at Conair Sports. 01295 771 088

He can supply all the bits you need.

You might be able to get away with peening the head....

Nick

smallw1449
19-Jun-11, 01:06 PM
Give me a call I keep pattern pistons and gaskets for the 503

Keith
0779 3807073

Al
19-Jun-11, 05:12 PM
Can someone tell me what size rebore I can go over the standard 72mm and still keep within the 503 regs?

I have sorted the head with a ball pein hammer & emery cloth. The head wasn't as bad as I first thought:) "shame the same couldn't be said about piston & bore".:(

I am also looking this time to fit twin carbs. Has anyone got a good set of Bings & inlet manifold for sale at reasonable money?

Pictures below of my piston:eek:

Any ideas on how to stop this happening again, as I'm not 100% sure at the moment what caused it.

PTO side piston was damaged, mag side piston perfect.
Had previous rebore, as good piston says 020

Motor number is: 3470470

Single carb
Fan cooled
Fan belt tension ok & no signs it has been slipping,
Plenty of space around the engine for air circulation
heads & barrels the correct way round

6184
6185

Al
19-Jun-11, 06:25 PM
Below are a couple pictures of the mag with the flywheel Taken off.

How much hassle is it to set the timing back up on these engines? I am thinking of marking the stator plate & crackcase in a couple of places, so I can split the bottom end to check & clean it out properly and then replace lower crank seals.


The wet rust stain is where I chucked a boat load of WD40 at it to try & help the flywheel to come off.
6186

6187

yamah
20-Jun-11, 01:41 AM
It looks like a fuel mixture/load (like detonation), high constant load and heat related failure, lubrication issue also looks likely. I am not closely familiar with the 503. Twin or multi cylinder air cooled 2 strokes often require different air/fuel mixtures for each cylinder to keep the plugs the right chocolate colour, from looking at the piston pic, I would think it's plug would show a lean condition.

Time at a different throttle setting before pulling the plug can mislead the colour analysis though. It has to be immediate after a short spell at a fixed throttle position to check colour for that throttle position mixture.

Twin carbs will allow the different settings to get each pots mixture precise for that pot, differences in carb settings can be quite extraordinary for a novice. Look at a Throttle setting Jetting chart for whichever type of carb being used to see which jets are working or combining at which throttle settings so you can select or adjust to colour the plugs correctly at a few different throttle settings through the range. This will help the output through the range as well.

A synthetic or part synthetic lube oil may also be a benefit, JASO FC or API TC or better.

It is not necessarily space around the engine that counts, rather flow INTO and through and across the fins and a clear exit path. If the cylinders are running at widely different temps after getting the mixtures right, there is a case for looking at ways to even up the cooling by tweaking airflow through the cowl. It could as simple as a small restriction across the cooler pots exit to make the hotter pot less relative air resistance and so get a higher share of the air. Extending the inlet throat of the cooling fan in a conical Air Ram fashion (or Kort's Nozzle style) may increase the air flow.

This is an afterthought.... the main fan may be depriving the cooling fan of available ambient air flow, this could be exacerbated the higher the rpm goes.

yamah



I am also looking this time to fit twin carbs. Has anyone got a good set of Bings & inlet manifold for sale at reasonable money?

Pictures below of my piston:eek:

Any ideas on how to stop this happening again, as I'm not 100% sure at the moment what caused it.

PTO side piston was damaged, mag side piston perfect.
Plenty of space around the engine for air circulation
heads & barrels the correct way round

6184
6185