Hi Guys,

Lowsided my MC21 back in February and got a nice broken collarbone. Bent my Tyga tail, forks twisted, fairings, mirrors and blinkers munged.

After moving house and neglecting the bike in the garage for some time I actually pulled her out last weekend and replaced her bent tail, pulled all the dead fairings off and had a look at rebuilding a spare set of SP forks. Just before I put her away I couldn't help myself. I had to hear her run.

Started her up with a little trouble (battery rather dead) and god she sounded mean. Heaps of smoke which started to clear, but then, oh oh, she's spraying coolant out of one of her pipes. I guess the knock was a bit harder thank I thought - at least one blown head gasket.

So I move onto my spare engine under the house. I bought a parts bike specifically so I could have a spare engine ready to roll. Nice Dry clutch vs my old wet clutch. Although compression was good I figured I should leak test it. I finally managed to fashion a leak tester out of a piece of PVC pipe, and old fuel pressure tester and a tyre valve. For an exhaust plug I used a generic sink plug and a flat piece of metal. Seemed to be a good setup, what the air compressor on the valve and start pumping.

Nothing.....

Strange, when I held my hand over the end of the tube the pressure went straight up so can't be a problem with those seals. Checked the bath plug, seemed ok. Spark plug in tight. When I took the expansion bottle off the top of the carb boot and blew into it - a wierd whooshing sound. When I sucked it almost held a bit of pressure. That can't be good I thought.

Blocked up the other side with my rig. Needle on the gauge almost moved when I pumped. No whoosh sound when I blow into the bottle hole, slight resistance to blowing and sucking.

Hmmm. Does my rig just suck? (no pun intended)

I finally give up and grab the 2 stroke fuel tin. Rip off the carb boots and reed valve cages. Poor some fuel in the left crankcase......

A waterfall of fuel pours out from behind the flywheel! Now that was unexpected!

So I guess I'm pulling down this motor before it goes in the bike. At least I have a leak tester now.

So as the thread title suggests, how do I know if I need a new/remanufactured crank? It moves well and looks ok.

Can I just split the cases and replace all the dodgy seals?

Should I do the top end at the same time? What's the minimalist way to go?

Cheers

Clancy

Might be less work to do on the original motor!

But I want my ring a ding ding dry clutch!

Also, the original motor has issues of its own - most likely leaking cranks cases as well.....

could be just the seals, or could be the crank or both.

Either way you will need to pull the engine apart to replace one or the other or both.

Measure the play on the big end to see what you got. From there you shoudl have a better idea about the crank.

sounds like the center seal. your attempt to pressurize one side by itself, or the other, ends up with no resultant air pressure. yet when both sides are sealed pressure then builds, so the leak can only be from the center seal. if the left and right seals leaked then you would have leakage when both chambers are filled with compressed air. am i reading your description properly?

and if it is the center seal the only way replace it is to split the crank? which i understand is best left for the professional with the experience and proper tools. and center seals are hard to come by?

or buy a new crank.

how long will a center seal last just from aging? mine is 20 years old and i believe i have the original in mine due to low mileage and while it is ok now, i have to wonder just how long before the effects of time cause it to fail.
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MC21R9N track
MC18R2J track
CBR954
CR250

I didn't actually have enough kit to fully seal both chambers at the same time.

I know the left chamber is leaking because fuel gushed out of it from behind the flywheel. The right chamber is also leaking, but not quite as much. I didn't bother to investigate further since one leak is enough to require a tear down.

So I can measure the runout on the crank, but if that's ok, is there any way to know if the center seal is going to be ok if it goes back in?

You will have to split the cases to change the outer seals anyway so I would just fill the outer seals with silicon and run a few more tests to see if the centre seal is leaking. Chances are it is leaking and even if it's not, how much longer will it last if reused?

I personally would have the crank rebuilt if you plan on keeping the bike.

Must say that I did notice you were not posting but still checking the forum for the last 6 months. Hope your all healed up now.

Andy

Bandit7

Healing wasn't really the issue. Renovating and moving house took most of the time. I also got a little Honda Z50 to play with which kept me busy for a little while.

Back into it now it's warmed up.

Do you know how many miles / kms the crank has done? They won't last forever, even if you renew the 3 main bearings (centre one with integral seal) with a crank rebuild, you also have the big end needle rollers to think about, and eventually the fatigue life of the con-rod. What you should avoid at all cost is splitting your cases, renewing just the Left & Right seals, then discovering a short while later a much bigger crank job is required.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

I thought a crank rebuild did involve new big-end bearings and thrust washers, at least in Australia anyway. There is a place over here that rebuilds the whole crank with the only original parts being the webs, rods, pins and outer drives if they are indeed reusable. I wont mention their name as I can't remember if we can say their name here or not.

Andy

Bandit7

I've known crank rebuilds to only replace the main bearings, but yes, a complete rebuild would also replace the big end bearings.
But there is then the overall cost to consider.
Set of main bearings at AUS $170, 2 big end bearings around AUS $200, crank seals around AUS $60 plus rebuild labour charge, maybe AUS $200.
That is over AUS $600, TYGA sell a full bottom end kit (brand new crank & all seals etc) for AUS $900.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

So really, what everyone is saying is that if the crank is of unknown age, and it has air leaks, and you're splitting the cases, might as well spend the extra money and get a brand new crank?

In the real world, we've found the main bearings go much sooner than the big ends in most cases. There was no measurable wear in the big-ends on my crank, and the motor it went in to after it had new mains fitted has done thousands of miles since (although it was unused this year!).

If money is no object, or you can measure the wear in the crank pins, then a new crank is a must, but don't be persuaded into a new crank if a set of mains will do the job perfectly well.
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Andy.
NSR-WORLD.COM

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What about a leaking centre seal?

The centre seal is part of the centre main bearing on the R5/6K, MC21, and MC28.
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!