IS THIS A STOOPID QUESTION? are cranks interchangeable?

[stirlinggaz]

right this is the 3rd time i have tried to post this,~
the long & short of it is : my mate has a knackerd trail bike & fancies a hoon around the local tracks.(with me on my mtx)
problem is his bikes fucked.
con rod is looking mashed up at the point where bits are pressed together..
i could send it for a con rod renewal, but tbh, the rest of the crank is in a bad way. the tapers all gouged & scored (presumably from when the woodruff key sheared off.

heres the solution i have come up with.
i have a bottom end off the exact same bike in the shed!
the con rod is good, so are the main bearings & crank.
would it be possible to transplant the the bits from my old bottom end (as its in bits, lol) into his knackered bottom end,
then use his decent top end.
would this be doable for an amatuer like me?
bearing in in mind i have restored a few bikes, done a few top end rebuilds but have have never really touched a crank or con rod.
any suggestions?
cheers,
GAZ

[finpos]

Can be done, but putting a load of worn parts into a differently worn engine generally doesn't get you a lot of long-time reliability. And that's if both the engines were in half decent condition in the first place!

[Raffles]

If the bits that you are transplanting are in good nick then go for it. It's only the same as buying an exchange item from a dealer.
____________________
A good loser will always be a loser.

[Frost]

Provided you put the crack and rod in as a unit it's definately do-able by someone with enough tools to open up an engine

As others have said your mates engine probably has worn bearings where the crank sits, so changing these would be inexpensive and a bloody good idea.

[stinkwheel]

Pretty simple. Pretty much a case of split the engine, lift old crank out, drop new crank in.

Obviously you'd need to remove whatever is on either end of the crank too which can be more or less complex depending on the model.

As said, might be a good time to renew the main bearings. Also fit new crank seals (assuming it's a stroker).

Look at it this way. You currently have two fucked engines. If you make an almighty arse of the job, at the end of the day, you would still have two fucked engines.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stirlinggaz]

hi,
thanks thats exactly what i want to hear!
if it is a case of just swapping one crank for another, then im sorted.( i think)
i've got nothing to lose, as both bottom ends are fucked anyway.
as said before, main bearimgs LOOK good, but if they do have to come off + be replaced, i presume im gonna have to get my hands on a special kind of bearing puller/slide hammer? (or bribe someone who owns one, lol)
cheers,
GAZ

[hmmmnz]

check the new crank for up and down play with the con road, if there is no movement then go for gold,
if there is movement then you need to get the crank pressed apart,
and then you'll need a new crank pin and big end bearing,
normally you'd do the whole sh' bang annd normally when you buy a conrod kit you get the crank pin, big end bearing and the conrod,
you'll also have to get everything pressed back together and balanced, just a little bit out and she'll vibrate herself to bits,

ive done it all by hand before, with just a drift and a hammer,
then a couple of bits of wood and a nail to balance.
that isn't recommended, and the bike was in a very low state of tune,
____________________
the humans are dead
I kick arse for the lord
Wiring Diagrams BIDNIP it bitches

[stirlinggaz]

hi,
not sure if im following you. (forgive my ignorance)
the conrod (on the damaged one) moves freely.
i can move it round/though the weights.
but its grinding at the part where bits are pressed together.
so in an ideal crank + con rod, what sort of movement should there be?
i dont have any sideways movement, well no more than a mil or 2.
cheers,
GAZ

[hmmmnz]

[stirlinggaz]

right,
looked at the replacement crank.
looks fine to me, but im no expert, lol
why do i have to replace the main bearings?
they look good, no grinding & no debris in there.
i know the bearings are only a tenner each but im gonna have to pay someone to remove them (or cut them off) & pay someone to put the new ones on.
why cant i just slap it in, replace the seals & hope for the best?
its a 20yr old engine, will it matter?
ps just realised the crank in the mtx125 engine is identical to the crank in the mtx200!
cheers,
GAZ

[Theoutsider]