Crankshaft Problem

I recently got a crankshaft re-built for my GT750 (kettle) when I re-fit it
into the lower crankcase it wont seat properly.
I have tried removing (temporarily) the locating pins that the main bearings
fit into and also removing the clutch and mainshaft assembly leaving only
the crankshaft in the lower crankcase.
It may be I have been working on the bloody thing too long and I'm knackered
but it would seem that that crankshaft or crankcases aren't true. It sits
nicely on the left hand side but is about a quarter of an inch off at the
right.
Anybody any thoughts or had any similar experiences.

R
Gt750 Red, Gt750 dead
--

 

It does indeed sound like the crank isn't true.

 

I would agree but..... it was built by SEP (Kegworth) I have used them
before and had no grief, they seem pretty professional.
The crankcases aren't the ones the crankshaft came from which is a potential
issue, as you know kettle ctrankcases get rogered if the chain snaps and
makes its exit skywards which is what fucked the original ones.
Is it possible for cases to distort?

I'm off back to my bottle of wine!!


R
--

 

Not to that degree, not without actually breaking, no.

Remember that built-up cranks are surprisingly easy to knock out of
true. They're only trued up with some light taps with a hammer, after
all. Could it have got clobbered in transit?

 

Without doubt ,possible it was well packed in polythene in a pile of
newspaper shavings., in a cardboard box.
Plan B is to try it in the original set of crankcases to see I get the same
result, if so it's going back.
I noted an article on SEP in last months CB I wondered if it was my
crankshaft in the photo.

Thanks for your support, it's keeping me sane!




R
--

 

Could be an ugly scenario: rebuilt crankshaft to fit different crankcases.

Why did the crankshaft need rebuilding?

And why were the crankcases replaced?

Best,
doc

 

The crnk got re-built as the seals were fucked and one of the rods/BE's
also. The crankcase it came from was knacked as the chain had snapped and
made its exit skywards, quite common problem on a kettle.
I am aware that swapping crankase halves is a no no but what I plan is to
see if the bloddy thing sits in the original lower case and try to establish
what is wrong, I also have another crankshaft which has fucked seals but it
will do to see if the case has distorted.
I note what TOG is saying that they would break if distorted by quarter on
an inch.
In the meantime I am writing a hundred lines.

I must not buy any more SOBS
I must not buy any more SOBS....................

Cheers

ras
--

 

With respect, I think I'd be taking the crank and cases you want to use
along to SEP and get them to check this out, rather than puddling about
yourself. The crank must be checked by setting it in vee-blocks and
measuring runout with a dial gauge, and the cases should be checked using an
accurate straightedge and feeler gauges.* I've never had dealings with SEP
but any of the engineers I use would regard doing this as a professional
courtesy.

The trouble with doing the try-fit routine you are suggesting is that with
the best will in the world there is some risk of making the crank
out-of-true even if it is not now, and I think that would invalidate any
warranty you have from the engineering shop. As well as that, the
consequences of things being out even by a very small amount could be
expensive.

BTW it's not that uncommon for alloy castings to warp if overheated-- do you
know the history of the cases you want to use?

Good luck

R

*Of course, if you have a fully equipped engineer's bench, then you'll be
able to do this yourself.

 

Thanks for yor reply, since my last ramblings on this I have tried the crank
in another set of cases and it fits, so... it looks like the cases are
shagged which is an arse to say the least having just re-built and
re-installed the selectors and water pump.
In response to your do you know the history of the cases the answer is no
and probably where the danger of 30 year old SOB's lies, there is nothing to
say that any of the cases are a matched pair.
I would have no idea if they have been overheated but without doubt they
will have been abused in some manner or form in the last 30 years.
I would agree that getting SEP involved is the right road to go down the
only problem is they are at Kegworth and I am in Edinburgh which makes it
errr tricky to say the least.
It would appear I'm up shit creek at the moment and don't know where to
turn, my feeling is that I could send the crank back and get it verified as
still true. Regarding the cases I don't know the best way to proceed.
Is there any way of machining the cases back to true?? or will it be a case
(excuse pun) of try until it fits, I have one remaining set as yet unopened
but utterly utterly filthy set.

Cheers,

Ras
--

 

Rasman99 composed the following;:

utterly utterly filthy suggests they might not have been touched for a
while, so whilst mucky, they might actually be good cases. I had this with
a Suzuki TS many moons ago .. three sets of crank cases and the filthiest
were the best ...

Good luck.

 

Given the geography, any automotive engineering shop should be able to check
the crank for true for a minimal fee if any..... There used to be plenty of
engineering shops in Embro, ask about. I used to use a guy in Haddington,
dunno if he's still there.

There's nothing that can't be fixed as long as you're prepared to throw
enough money at it! I'm not intimate with kettles specifically but in
principle there are two ways of doing that 1) linebore the cases oversize
and fit oversize main bearings, 2) build up the journals with weld and
rebore to standard. Which would be applicable in your case would need to be
assessed by an engineer. We are talking crankshafts here, but you better
assess whether any other shafts running in the cases are also running out of
true, and whether this is going to cause problems (if they are.) Remember
also that if the cases are warped you'll have to skim the cylinder mating
face true as well, otherwise you will have endless trouble. If you're
contemplating this sort of work you really should develop a relationship
with a local shop (and have deep pockets.)

or will it be a case

I'm pretty sure that will be the cheapest route, even if you end up having
to try several sets of cases.

Cheers R

 

Neither of these will work, as it's a two-stroke and therefore has a
built-up ball & roller crank, not plain bearings.

 

Beat me to it there TOG.

I am opting for the pick'n mix method from my ever growing SO kettle spares
stock.
I was also going to try assembling the crank into the cases and very lightly
tightening the 14mm main bearing bolts to see if it will seat.
It is difficult to juggle with the seal lips and getting the mains to locate
in the pegs due to the weight of the whole assembly as attacking it with
weaponry will no doubt wreck it.

Back in a bit

Ras

--

 

Just to be precise about this, yes at least one of them will, as we were
talking about the cases, not the crank. There is no reason at why the cases
can't be built up and remachined to accept the outer bearing cages.

 

Yes, you're right; this could be done. Never heard of it being done,
mind, but somebody must have.....

 

....probably using isopon...

Mind, I once succesfully repaired a timing cover with isopon, albeit on a
Vauxhall Viva.