Gg Engine Rebuild

[magicmat]

Right,

Im rebuilding a 1993 GasGas. (but been in a box for 2-3 years) Its my first Monoshock trials bike. Being only 20 and at uni i have only had a couple of old vintage trials bikes which were cheaper and simplier.

Anyway, its in a box and im rebuilding it, but im alittle miffed to what in the engine i should rebuild.

From taking the barrel apart What i planned was :

-New piston and rings

-Bore rechrome

-little end bearing

-big end bearing ?

But.... What else should i change?? the GB was alright. The clutch plates did have alittle drag, but nothing serious.

Do i change

-The conrod

-The main bearings??

-Gearbox bearings??

Is there anything else that i should be thinking of ??

Thanks for your help in advance !!

Mat

[steve shakeshaft]

Personally, I would never dream of opening up an engine without replacing all bearinmgs and seals. Your attitude may change depending on whether the bike is a keeper or not. I would definately not overhaul a crankshaft on a 2 stroke to change a big end without mains an crankshaft oil seals.

[arieltt]

Matt,

I guess that it depends upon the condition of the bits that you have.

Is the chrome on the bore shagged (deep scores)? If not then leave it alone. If the piston is OK you might just need new rings. Is there any play in the big end (can you move the conrod up and down on the bearing)? Do the main bearings rotate smoothly (even a hint of roughness means they are knackered)? I doubt that you would need to change the conrod unless its bent. New crank seals would be a good idea.

Really its hard to say without seeing the bits but some of the above will hopefully help.

Cheers,

Tony

[magicmat]

Cheers for the reply,

i will change the crank seals,..I changed the drive shaft seals on my mini this summer, and the crank seal on the engine, what a job.

So i wouldnt ignore stuff like that.

I will do the gaskets and stuff. But when i look at what other ppl have done they just seem to stick a new piston in and a little end.

Im at uni at the moment so cant check the bore. but the scoring on the piston was very severe,..i mean lots of it, and the bore too, but i cant remember if it was deep.

(will check when i go home)

Initially i was going to change every bearing in the entire engine, but then thought whats the point not like its done 100,000 miles??

But it is for keeps, its to get me knowledge about the internals of the bike, and i may let my GF use it, or younger brothers in a couple of years

The bike was a poor runner, but im putting that down to poor spark and compression. So a magneto rewire is on the list.

2 Qs.

-anyone give me an idea for cylinderhead torque setting?

-why on earth is there no head gasket and just o seals??? i have ideas but thought id ask.

Mat

[magicmat]

btw.

The Gearbox was fine,

but should i change stuff in there??

Or maybe i should crack the engine open and check.

Would i have to split the engine to change the crank seals..

So many Qs.... sorry

[arieltt]

Matt,

Yes you will have to split the cases to change the crankseals. Main bearings aren't expensive so why not change them as well while its apart. I'd check the gearbox bearings and the condition of the cluster and maybe replace worn bearings where necessary and the oil seals anyway.

Tony

[magicmat]

By cluster do you mean the Cluctch and that basket??

The o rings between the bore and head seem fine but will change them,

But what is with the lack of headgasket?? this is the first post 1983 bike ive taken apart and the older ones have head gaskets thats all.

is it because of low compression ratio???

[gasgas]

Hi, a lot of bikes/cilinders use o rings nowadays. The positives are good sealing and good heat transfer. I wld say that heat transfer is the main reason for the o-rings usage.

The negative side is the fix deck height.

With gaskets you can alter the squish height rather easy.

I think Arieltt means the gearshifters, not the clutch, but correct me if i am wrong.

With a 1993 bike, i wld change any bearings/seals/components which are doubtfull. It has seen a lot of hours and when you want it for keep just make it completely fresh inside out.

Happy wrenching

Edited November 16, 2005 by gasgas

[magicmat]

cheers for the help guys,

gonna get my bore looked at and a prive for the chrome.

Then will split, and change the main bearings, little and prob big end (if not expensive) and ill check out the gear box to see if there are any collapsed bearings or the selecter forks are bent.

Cheers !!!

will let you know how i go

Matt

[magicmat]

hey,

ive ordered all the parts now,

-new piston

-bore replate

-main bearings

-crank seals

-little end

-Gasket kit

-Manifold rubber (mine split)

-Magneto rewire

-and thermostat (as someone wired the radiator to always be on, so guessing thats the prob

-BUT the big end, the guy at <mention of this company is not permitted on Trials Central> wasnt sure if i could buy the big end by its self, said i may have to get a conrod kit cost

[magicmat]

but then i dont potentialy get the dowel,

which is like having the crank ground,

i dont know if you even get this 'dowel' with the kit !!!

i want this bike to last 3-4 years so will the bearing be enough? or go for the kit,

Cheers

[chris]

Hi

I'm guessing you won't be changing the big end bearing by youself so why don't you ask the shop who you intend using to have a look, if there any good they will tell you if you need just a new bearing or not. No one can tell you if you need one without looking at it.

You do know the crank has to be pressed apart to fit the new bearing then pressed back together and realined to less than a thou, quite a specalist job.

Chris

[magicmat]

Hey,

thanks for the reply, i took the engine to the shop, and they said the bearing was gone, and they would get one in, thing is instead of being

[chris]

Magicmat

I've seen a crank realigned in a lathe before, using Dial Test Indicators (D.T.I.) to check the alignment and a big mallet but this is probably not a right good idea unless you know what your doing. The main bearing journals on each side of the crank have to be concentric, not sure of the actual tolerance but if it's not right the main bearings will fail prematurely and probably other components too, can't really see how you could use a diy laser.

Have a read of this website it explains how it should be done properly www.ericgorr.com/twostktech/2techcrankrepair

He also meantion in there that the connecting rod, bearing and pin should never be reused.

Chris

[magicmat]

hmm.

had a chat with my old man, hes a fabricator/welder/engineer dude,

And he recons that we could true the crank in the lathe using D.T.I's as we have them from aligning motors and gearboxs in heavy machinary etc etc.

But it will take alot of checking and messing around to get it right. and rotating the mass on the crank pin? not the ends of the crank.

I couldnt use a lazer, i was just beeing doopy, as im doing my finals at uni and brain is like s sponge

I had a good read of that website, very good info, he definately knows what hes on about, i will seek costs of having it professionally done but if costs too great ill enlist the old man.

If i was to do it, the problem i would encur is at present i dont know the tolereances between the bearing (or thrush washers) to the crank. hmmmmmmm....

Looks like the rod kit will be a sound investment, but the connecting rod does need to be changed within MX bikes which he talks about but i dont know if it is as true for trials. i will have a chat with the shop and find some engineering firms,....unseen costs are starting to rise ...

The advice is greatly appreciated (printing off the guide !!)