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503 rebuild


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Calling all Rotax 503 experts. I'm in the process off rebuilding my engine and, being a very early(si/sc), it has main bearing plugs in the upper crank-case half. These have a step on them as to indicate a fitment orientation. Anyone know how they should be aligned?

 

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Also, when sitting the crankshaft in place, the O-rings (64 x 2) get pushed up so as to be too big

 

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Any advice? When I stripped the engine, only debris from earlier rings was present.

 

P.S., engine has still to be finally cleaned so I know it not ready for bolt down.

 

 

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Many thanks Jason. It seems that my engine is a mixture of old and new. i.e., provision for rubber plugs(early models) and o-rings(later models). The step in the plugs must be due to wear as there's no mention of orientation in the manual. With regard the o-ring fitment, seems I've got the wrong size in a couple of places. I thought they were all the same size. I've been advised that "most people" leave them out and seeing as I have the plugs, albeit a bit worn, I'll rely on them. It would appear that they're there only to prevent the bearings from rotating in the housing rather than providing compression sealing between cylinders or the outside world.

 

 

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  • 1 month later...

Sing along now..."What a difference a day makes......." de-coking my pistons. 24 hours soaking in WD-40 and a light scrub with a pot scourer. I said LIGHT......OK!

 

According to advice found on the net, there should be markings on the top to indicate which side faces the exhaust port. I can see some id marks but nothing else. How do I determine the correct way to refit them?

 

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Tip.... if fitting rubber o-rings or seals, use pbr rubber grease (a tube will last a lifetime) if lubrication is required.

 

Do not use crc, wd-40 or mineral oil/grease.

 

The rubber can swell, become difficult to fit and the life span reduced.

 

 

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Thanks Nev, thanks advice at the right time.

 

Downunder, your advice has come too late. I've already reassemble the crank case using WD-40. Not going to take it apart again.

 

 

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  • 3 months later...

Can I swear and swear and swear? No? Damn it. In my post #11, I said I'm not going to take it apart again. Well,whilst re-mounting the engine on it's base plate this morning, I suddenly realized that I'd mounted the cylinders and heads back to front. ie, carburetor on the starboard side, exhaust on on port. How I came to do it I've know idea as I had marked the parts when I took them off. FWIW, it went together no problem.

 

 

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Please ignore my last post. I'm going mad. After taking a break I went and had another look. It's right after all Thank God. Maybe I got out the wrong side of the bed this morning.

 

 

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Thanks guys. My next problem soon raised it's ugly head, refitting the gearbox with the few studs I have left over. Don't think I've lost any of them but there's more holes than studs. I looked up views of an exploded engine and got totally confused as to what type of gearbox I have. I initially thought it was a type B but then changed my mind to a type A with an adapter plate. ?????????

 

DSCF4672.JPG.6423a83f3746140421e2d065c0a38010.JPG Engine as found.

 

I'll play with it again today after I remove the recoil starter assembly. I found the magneto securing nut and washer still on my bench last evening.

 

Ya know, I always thought I was a careful planner and orderly in stripping engines (I've done quite a few early types when home maintenance was possible) but this is my first 2 stroke. Simple....yes, but I'm confused by that simplicity.

 

 

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MESSED UP!!!, I didn't mess up, I had a slight brain fade. It'll happen to you when you reach my age(76) Not having a go but there's no way I'm pulling it apart again. It's all there somewhere, just got to get it into the right order.

 

 

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It'll happen to you when you reach my age(76)

Don't worry mate my Grandfather used to say " your only as old as the woman you feel".

I suppose aircraft count as a female ! babe.gif.ba7bd836fca21da284bb7eb1736ad73c.gif

 

So your still young!

 

 

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The "A" type and the "B"type boxes are, as far as I know, identical in size. The difference is mainly in the no. of Bellville washers exerting pressure on the clutch. I had the modification done by Floods years ago.

 

 

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When I say "clutch" I am referring to the coupling which consists of 2 mating inclines which is spring loaded by the bellville washers. This is designed as a simple slipping coupling to minimise transmission of power pulses to the propeller which would act as an inertial flywheel. Called a dog clutch I think. (Ahem...) If you can get my drift. Also, the "A" type box had a rough, sand cast finish: the "B" box was smooth.

 

Just found this link to Youtube also.

 

 

 

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