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RS125 engine rebuild

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Pete.
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Joined: 22 Aug 2006
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PostPosted: 02:29 - 04 Feb 2011    Post subject: RS125 engine rebuild Reply with quote

Just finished rebuilding Paddy's engine and I thought I'd put up a few pics and some info that might help others. This engine was the one that seized, turned out a main bearing had collapsed and bits of the cage had wrecked the barrel and piston.

Firstly I have to say the D&K gasket set was crap, I got it fitted but nothing like the fit of a genuine Rotax gasket. Lining up holes was definately a 'best compromise' affair and I had to trim bits off to stop it fouling stuff like the waterpump impellor.

Also, the Mitaka cylinder was very tight on the studs. I had to drill the base stud holes out to 8.8mm to get it to slide on comfortably. The instructions on how to choose a gasket were very misleading too, more on that later.

Right, to business, strip down the engine, clean out all the crap and check all the parts I have. It's important to clean up not only the inside but the outside of the engine thoroughly, to stop dirt off your hands getting in when you re-build.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Pete.
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PostPosted: 02:35 - 04 Feb 2011    Post subject: Reply with quote

Heated the bearings up to 80 degrees and slid them on the crank, then heated the crank case up to fit the crankshaft into on side. note the gasket, not a great fit. I had to glue it to the first case half to make sure I could get all the bolts in when I come to join the cases.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Pete.
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PostPosted: 02:42 - 04 Feb 2011    Post subject: Reply with quote

Next I assembled the gearbox, shift drum, shift forks and balancer into the cases, fitted the second bearing on the crank by pre-heating then did the business on the second engine casing, heating it to 100 degrees then fitting over the first. Noted that there was quite a bit of drag on the crank, enough to make me pull the casing off and re-check the clearance. With the cases split there was still a good bit of drag on the crank so I figured it must be the crank seals.

Once the cases were all buttoned up I fitted the piston without rings and then the barrel (after drilling out the holes so it would fit). The reason here is to check the squish clearance. In the Mitaka instructions it said set the squish to 1mm by swapping gaskets. I fitted the thickest gasket and put some playdough on the piston, fitted the head and turned the engine over. The playdough never got squashed at all. Tried the thinnest (0.3mm) gasket, and I could only get the squish down to 2mm.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy


Last edited by Pete. on 03:00 - 04 Feb 2011; edited 1 time in total
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Pete.
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PostPosted: 02:58 - 04 Feb 2011    Post subject: Reply with quote

Now clearly something doesn't add up. The installation instructions say shoot for 1mm squish and the lowest I can get is 2mm. I checked the manual, and they don't mention squish but describe how to measure the compression ration by filling the combustion chamber with 2-stroke oil. I found a inkjet refill syringe and measured the combustion chamber at exactly 11cc, giving a compression ratio of 12.35:1 - almost perfect (spec is 12.5:1 +/-0.5).

If I had somehow managed to achieve the 1mm of squish the Mitaka instructions called for, I would have ended-up with a compression ratio of 14.26:1. Far too high.

Incidentally, if I had followed the 'safe option' of using the thickest gasket of 0.8mm, the compression ratio would have been just 11.58:1 - below the minimum specified. The engine would have run ok but not been as sharp as it was supposed to be.

Anyway, having sorted out the clearances I built up the top-end, drilled out a broken bolt on the starter and drilled & tapped the two bolts that hold the ignition sensor, buttoned up the engine.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Pete.
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PostPosted: 03:03 - 04 Feb 2011    Post subject: Reply with quote

Oh yeah - Paddy - your engine's done Smile
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Paddy.
Red Rocket



Joined: 14 Feb 2008
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PostPosted: 10:52 - 04 Feb 2011    Post subject: Reply with quote

Pete. wrote:
Oh yeah - Paddy - your engine's done Smile


Wrong guy, that engine looks clean... Mines this dark shade of crap..

Wow.. Sorry it caused you so much grief, the genuine gasket set was put of stock at PJME and the eBay ones were 'genuine' but feedback was less than desirable.
I think the mitaka kit says something about drilling holes out, but I didn't really pay much attention as I knew I couldn't fit it Laughing

A massive thanks to you, please let me know your fave drink or something...you have done me a massive favour! Karma Thumbs Up
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Kickstart
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Joined: 04 Feb 2002
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PostPosted: 11:11 - 04 Feb 2011    Post subject: Reply with quote

Pete. wrote:
If I had somehow managed to achieve the 1mm of squish the Mitaka instructions called for, I would have ended-up with a compression ratio of 14.26:1. Far too high.


Possible that they haven't rewritten the instructions between the 122 and 123 engines.

With the 123 the theoretical compression ratio was between 12.5 and 15.3 depending on the model (all +/- 0.5).

All the best

Keith
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Pete.
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Joined: 22 Aug 2006
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PostPosted: 19:03 - 04 Feb 2011    Post subject: Reply with quote

Kickstart wrote:
Pete. wrote:
If I had somehow managed to achieve the 1mm of squish the Mitaka instructions called for, I would have ended-up with a compression ratio of 14.26:1. Far too high.


Possible that they haven't rewritten the instructions between the 122 and 123 engines.

With the 123 the theoretical compression ratio was between 12.5 and 15.3 depending on the model (all +/- 0.5).

All the best

Keith


Possibly, the instructions were title 'AF1', whichever engine that is.
Paddy's took his engine now to be fitted. The premix pump is working just fine and I've primed the fed tube so once it's pumped the tubes clear of bubbles he can fill the tank with fresh juice to lose the premix.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 19:10 - 04 Feb 2011    Post subject: Reply with quote

Pete. wrote:

Possibly, the instructions were title 'AF1', whichever engine that is.


There were 3 main models of AF1 125. Early ones used the Rotax 127 engine (very different, no balance shaft, a motox engine detuned for the road). The AF1 Sintesi and AF1 Futura both used the Rotax 123 engine.

All the best

Keith
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Old Thread Alert!

The last post was made 7 years, 347 days ago. Instead of replying here, would creating a new thread be more useful?
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