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View Full Version : Number of miles until rebuild?



chainsaw
05-15-2004, 09:05 AM
What would one of the many experts here reccomend rebuilding the engine at? Especially if the poor beastie is flogged mercilessly? Or would my money be better spent or on big bor kit?

Micah / AF1 Racing
05-15-2004, 10:38 AM
When you stock cylinder is worn out you just replace it with a bore kit since the stock piston (oversizes of 40.6 and 40.8mm) is $89, the gasket set is $21, and the machine work to rebore the stock cylinder will be $30-50. You can bolt on a TP72 for only $199.

JonnyRS50
05-15-2004, 10:45 AM
over 25k km on orignal rings and was still going strong when i sold it, good compression, first time starter and all that cack if thats any indication for you to go by.

Steve / AF1 Racing
05-15-2004, 11:00 AM
In terms of the stock cylinder and bore, the life span of the RS50's top end is about 7,000 to 8,000 miles. Truly impressive for a two stroke motor. Of course some engines last longer, and some engines go out earlier. The cause of early failure of the top end of the AM6 motor is usually attirbutable to one thing. Heat. Heat will ruin the motor prematurely in one of two ways. One: not enough oil makes it into the fuel mixture, which lowers lubrication efficiency and thus causes the motor to run hot. Two: carb jetting changes create a too lean and thus hot burning mixture that will also overcome the cylinder and especially the piston ring's ability to dissipate heat. In either case the failure of the piston rings causes them to lose their sealing abiltiy and they eventually fail in one of many ways, most commonly they come out their seat in the pistion and scrape the inner bore (aka seizure).
"Flogging" an RS50 motor does no appreciable damage to the life span of the engine. However, the heat that can be generated by very hard riding can cause the above mentioned consequences. In general though, the AM6 is quite tough and will deal quite well with hard riding. A road going 50cc motor must be designed to be quite tough and the AM6 is certainly that.
I would suggest a big bore kit only if you plan to add the other common performance pieces with it (i.e. carburetor, and exhaust). The bore kits themselves are quite inexpensive and will markedly improve the bikes power, but not to the extent that you would see when combined with a carburetor and exhaust.
The big issue with the bore kit is that it will definately shorten the lifespan of your crankshaft. The AF1 staff tell me that the crankshaft is good for about 9,000miles on a street only RS50 that has been kitted with carb, bore and exhaust. The crankshaft cannot be replaced by the common craftsman wielding home mechanic and thus the replacement of the crankshaft is by far the most costly operation that will need to be done to the RS50's motor.
Alright, now that I've written you a novel.
Good luck
-Steve

Tom
05-15-2004, 11:02 AM
mine needs a rebuild, getting it done free :)

JonnyRS50
05-15-2004, 11:07 AM
In terms of the stock cylinder and bore, the life span of the RS50's top end is about 7,000 to 8,000 miles. Truly impressive for a two stroke motor

WTF, it will last way longer than that, my RS125 has done like 12k km on orignal rings and thats in a far higher state of tune!

Craig also got about 25k km on his RS125.


You must be doing somthing wrong like redlining it from cold lol

Steve / AF1 Racing
05-15-2004, 08:08 PM
I hope that you are right JonnyRS50. But I feel the need to note that you are referring to UK bikes and they, on average, tend to last considerably longer than US ones. I have yet to uncover any techinical reason for this discrepancy, but it is true that most US bikes are nearing the end of their ring's life span at about 7,000 to 8,000 miles. I've verified this with a number of the American tuners, and while a top end can last the 15,000 miles that you mentioned, it is still a rarity, not the norm.
Like I said though, if anybody reading this knows differently I'd love to hear about it.
-Steve

chainsaw
05-15-2004, 08:32 PM
Thanks for all the info guys. Much appreciated.

JonnyRS50
05-16-2004, 08:09 AM
I have just concocted a possible theory, in genral i fill up with 97 ron fuel, you guys in the USA seem to have concidrably lower octane fules so pre detination will happen alot more therefore wearing out the motor quicker.

Possibly the reason, but then possibly not.

Steve / AF1 Racing
05-16-2004, 01:30 PM
The best octane rating that is commonly available in the US is 93, but in many parts of the country 91 octane is the very highest rating available.
I also wonder if the cooler average climate that the UK has is somehow related to the longer lifespan of the bikes that are over there.
To give you some idea of what I mean: The average temperature when I ride my RS50 is usually 80-85 degrees F (or 27 to 29 degreees C). I'm guessing that you commonly ride in temperatures that range from 15-20 degrees C.
In general the cooler the weather the cooler that the bike will run. This could be part of the longevity issue also.
-Steve

gregoire2k3
05-16-2004, 02:16 PM
good theory...i use Shell's best petrol ( cant remmber what it is called ) but im sure its 98 octane but quite expensive. but still 7-8k? that cant be right at least 10k-12k standard :/

priller
05-21-2004, 04:43 AM
Originally posted by osors50
. The crankshaft cannot be replaced by the common craftsman wielding home mechanic and thus the replacement of the crankshaft is by far the most costly operation that will need to be done to the RS50's motor.

-Steve

Any reason why it cant be replaced at home?


WTF, it will last way longer than that, my RS125 has done like 12k km on orignal rings and thats in a far higher state of tune!

Rings on mine snaped at 10,000km and it aint redlined from cold.

JonnyRS50
05-21-2004, 05:27 AM
that could of possibly been a falt with the rings, such as a hairline fracture, or even the person who owned the bike before you gave it a hard time before you had it.

priller
05-21-2004, 05:58 AM
Originally posted by JonnyRS50
that could of possibly been a falt with the rings, such as a hairline fracture, or even the person who owned the bike before you gave it a hard time before you had it.

Didn't have rings at 8k so it was due for rings anyway.

I've had it from new, its 3 years and 4 months today.

JonnyRS50
05-21-2004, 06:05 AM
unlucky then i suppose, mine has 13k km on the clock on orignal rings as far as i know and its still going strong.

priller
05-21-2004, 06:06 AM
Your will be due for rings and piston soon then.

Mine does have a hard life though, I have no problems reving it past 12k when its warm.

JonnyRS50
05-21-2004, 06:11 AM
nah mine is fine and dandy, starts easy and has good compression, lots of life in it yet, atleast anolther 5k km me thinks

priller
05-21-2004, 06:15 AM
Mine still started fine even with a snapped ring, could park on a hill though, even in gear the bike would roll forwards:D

JonnyRS50
05-21-2004, 06:19 AM
lol put this into gear and you cant push it, well you can but like only about 1 foot till you hit the compression stroke hehe

priller
05-21-2004, 06:20 AM
Mine would keep turning over and over.

BTW aprilia recommend changing the piston at 16k.

JonnyRS50
05-21-2004, 06:25 AM
im just gonna see what happened, i know that key walshe got 35k km out of his mito on orignal rings so i know they can be pushed!

Steve / AF1 Racing
05-21-2004, 06:45 AM
I was told by two different sources (neither of them were my dealer) that the replacement of the crankshaft requires a few special tools. In addition to the special tools, the replacement of the crankshaft needs very close attention paid to some of the critical parts of the disasembly and reassembly of the motor. I do not know what those critical parts are specifically, but if you have ever looked at an exploded diagram of a motorcycles gearbox you can see that all those parts could make the job of replacing the crankshaft all by yourself a very difficult job. My guess is that setting all the forks, bearings, planetaries is a pretty delicate chore and if any one part is out of tolerance or position, the bike won't shift properly.
I imagine that with a good shop manual, replacing the crankshaft is possible by the home mechanic. But my preference is to leave it to the guys who do it all the time. Their labor costs won't be likely to exceed 170 pounds anyway, and most shops will garuntee their work.
-steve

priller
05-21-2004, 06:47 AM
Doesn't sound like any good reason I cant do it at home then.

JonnyRS50
05-21-2004, 07:27 AM
i bet the class a torque wrench as a special tool lol