Crank rebuild? Big-end bearing spec.

N

nokturnal

Member
Did a bunch of searches, came up empty. (I wish the search would allow 3 letter words!)

I was intstalling a new silencer (needed a SA) and thought I would reseal the exhaust at the spigot while I was there. I decided to take a peak at the piston, as the bike is new to me, and I figured i would need to rebuild the supposed virgin engine at the end of the season.

Lucky I did... the piston had obvious signs of a nip-up. I could easily feel them with my nail. Off come the top-end... what do I find but a badly scored bore, with a small piece of nicasil missing!! AND the piston appear to be a Wiseco, or something forged (anyone wknow the part number 15399-38?? It wa son the top of the piston). The rod is Prox (stock??) and had a bit of lateral play (no vertical play), feeler gauge says 1.03mm. The engine service manual reads that 0.8 is normal and 1.0 is the limit.

What do people actually USE as their limit? :confused: Is it like a KTM manual that recommends a vlave check every 2 tanks of gas? Or is 1.0mm realistically the end of the line?

I really want to get this turned around and back riding. I have a second kid arriving mid-june, and wnat to get some riding in before the whole place shuts down!
 
Not sure where you are from buddy, but I'm going through the process of replacing/ rebuilding my crank I have an 05 EC250 and I've never been deeper than a cylinder head.

But I'm learning all the time. I found a stock crank kit from gasgas is ?450 just the conrod and bearing kit is ?155+ pp from the uk. In new Zealand from www.motocrossparts.co.nz it's $155 about ?68 then in Bradford, there is a company that will do a complete crank rebuild including parts for ?120

I'm going to phone them tomorrow and get a better picture. They also offer a complete engine strip down for ?85 so theoretically I could drop off the engine and get a rebuild with parts for ?205 sounds like a deal to me.

http://m.roadanddirtperformancebikeparts.co.uk/
 
Just to complete this thought, for the purpose of future searches -

1. I have heard from a reliable source that 2mm is a better, or at least more realistic, limit for the big end.
2. Some bikes did in fact come with a Wiseco piston standard. Luck of the draw.
3. Pro-X is a stock rod

(2006 EC250)
 
Shouldn't it be 0.00mm play in the big end? The slightest movement and 2mm isn't that slight in engine terms, would be exaggerated massively as the forces press on it as it whizzes round at 10,000rpm?
 
KTMAndy said:
Shouldn't it be 0.00mm play in the big end? The slightest movement and 2mm isn't that slight in engine terms, would be exaggerated massively as the forces press on it as it whizzes round at 10,000rpm?
Click to expand...

My gas gas engine service manual says that .8 is standard. 1.0 is the limit. This is side to side movement, not vertical.
 
My guess is that your crank will be fine for awhile, but it never hurts to remove the ignition cover occasionally and try to wiggle the flywheel to check for movement. I forget, is it up and down that is ok, or in and out that's ok? Someone will chime in. Rule of thumb is if a crank is close to spec on any well engineered bike it'll last one more average season, or another year. If you ride alot, or keep it on the pipe most of the time at high RPM, caution is a good thing.

In the olden days, when I had to walk to school 5 miles one way, and 7 miles back, when it snowed in July, and you got sunstroke in January, and the hills got taller each year, when we had to use old newspaper to plug holes in our shoes, that is if we had shoes, where the cows had 2 short legs on one side, and 2 long legs on the other from standing on the off camber hills, you wiggled the crank. If it moved a little but not much, just do a top end. If it moved alot, put in a cheap set of rings and sell it to your buddy!


Fred Overstreet is fixing my blown top end as we speak. He knows what's reality and what's not. He just told me tonite on the phone, but my out of spec brain forgot already. Fred?...
 
jgas said:
Fred Overstreet is fixing my blown top end as we speak. He knows what's reality and what's not. He just told me tonite on the phone, but my out of spec brain forgot already. Fred?...
Click to expand...

per the GG service manual RANGE is .8mm-1.0mm side play on the big end
 
Note that the crank will have normal detectabe axial play on '01 to '07 GGs that use the roller bearing on the ignition side. Older use a ball bearing, newer a sealed ball bearing(dumb idea IMO).

My '12 has a fair amount of axial play on the ignition side, so it was either built with the open roller bearing because of parts issues at the factory, or the sealed ball bearing is bad and its going to blow soon.:eek: Neither would surprise me at this point.:confused:
 
I received my "real-life spec" from a very reliable gasgas dealer (who was not doing any engine work for me).

That said, I do not like to take risks on such things and plan to do the crank when this piston is done anyway.
 
GMP said:
My '12 has a fair amount of axial play on the ignition side, so it was either built with the open roller bearing because of parts issues at the factory, or the sealed ball bearing is bad and its going to blow soon.:eek: Neither would surprise me at this point.:confused:
Click to expand...

Or it could be the shaft tolerance is on the low side and the crank slides axially through the bearing (or the case tolerance is on the high side and the bearing is moving in the case). The big question is radial play. How much does it have?
 
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