Hi guys,

I've just done a top end rebuild on my mc21 and am unsure about how to test the centre crank seal.

Ideally i'd have tested the crank seals prior to doing the top ends, but it didn't work out that way, and now i'd like to establish some confidence in them before sticking the motor back in the frame and reassembling the bike.

It ran ok before the rebuild, was just low on compression. I've used the twostrokeracelabs leakdown test kit and the left/right crank seals seem ok (0.5psi loss over 6 minutes after charging to 7psi.

With the boost bottle connected to the carb insulator, am i not pressurising both sides of the crank case at once? Do i need to remove the boost bottle and blank off the hole for it in the insulator and then repeat the test on both sides of the crank to test the centre seal?

What type of seal is the centre seal, and should it seal perfectly?

Any help from the guys who've been three and done it before would be appreciated. This is the first leak down I've done on a twin and I don't understand the procedure 100%.

Thanks,
Scott

zillabot wrote:With the boost bottle connected to the carb insulator, am i not pressurising both sides of the crank case at once? Do i need to remove the boost bottle and blank off the hole for it in the insulator and then repeat the test on both sides of the crank to test the centre seal?

I'm not an NSR expert, but the if the boost bottle provides a link between inlets, it should be removed and the connections blanked off.

I found this youtube video useful first time round.

https://www.youtube.com/watch?v=zdq7NRcUIp8
_________________
If it ain't broke, fix it until it is.

Thanks.

I foolishly thought the boost bottles may have been connected when i was inside typing my first post above (god knows why I'd have done that).

They're definitely independently sealed, which means i have a major leak across the crank centre. I can't get any pressure in either crank case half unless the carb inlet of the opposite cylinder is blocked.

If i spray a spot of crc into the back of the reed cage, then pressure the opposite crank case half, i get instant bubbles showing at the apex of the reed cage. So the air is going straight across the crank centre somehow.

That might make sense if a labyrinth seal was used for the centre bearimg but i came across a thread while searching that suggested the nsr crank centre bearing uses a sprung seal, in which case little or no leakage should be expected.

Oh, and lastly, i found that i needed to seal up the crank oil feed port to be able to generate any pressure while testing. Seems normal enough but it's all new to me, and I've not seen others explicitly mention this in other leak test write discussions.

I'm thinking this means i need to split the cases and expect a replacement/rebuilt crank is on the cards....anyone care to confirm?

Thanks
Scott

Doesn't sound great, Scott...

As I said in my post, I'm not an NSR expert, but if the NSR have (oil) pressure fed mains, you'll also need to block those off to get an accurate picture of what's going on. Given the complete lack of pressure, I'm surprised the bike actually ran!

The centre seal will have a spring to help seal against the crank. However, over time the lip of the seal will go hard, especially if the bike has spent long periods of time stood idle, or has had fuel in the cases.

Fortunately for you, this is a great forum and there's a wealth of knowledge on here and I'm sure someone with specific experience of the MC21 will be along with some advice.

Regards

Mark
_________________
If it ain't broke, fix it until it is.

Thanks again Mark for your comments.

Surprisingly the bike was running quite cleanly. However, i only managed to ride it once after purchasing. I'm fairly sure the motor was dormant at a japanese wrecker for some time. The rear cylinder was down on compression (around 110psi) vs 130 on the front, and i could see some evidence of a prior seizure on the front cylinder through the exhaust port, so i decided to pull it down. That was 12 months ago, after which i became very unwell and was laid up in bed/hospital for over 6 months. With all that behind me i'm just getting back into it recently - i ordered some rebuild gear from tyga and threw it back together last week.

Thinking about the seal itself - i couldn't help but think of reasons as to why the bike may run well but the leak test go so miserably. Is it possible that the seal softens a little at operating temperature and improves its effectiveness? Or perhaps if a an area of the seal cross section was missing, thus allowing ample transfer of air during a leak test, perhaps at operatimg rpm the seal effectively becomes a solid disc and prevents most of the cross flow (like a spoked wheel in a crosswind).

At any rate i know I've got to pull it all down and inspect, I'm just trying to be certain that I absolutely need to given the bike was running decently. Of course that is no guarantee that the seal wouldn't deteriorate further and cause more damage down the track.

Thanks again for your thoughts.

Just to close this thread off, I've pulled the crank and found that a) the centre bearing outer o-ring had shrunken away to virtually nothing, and b) the centre bearing spun like a roulette wheel.

Left side bearing wasn't great either. Fun times!

Thanks again to Mark for helping.

Definitely time for a re-build, Scott..

When the seals on my NS400 failed, the bike ran fine from cold but deteriorated rapidly as it warmed up.

The real kick in my case was that the crank had just been rebuilt - supposedly with fresh seals, so it took a while to work out where the problem lay. The situation was made worse by the fact that just about everything (engine/carbs/ignition/exhausts) had been altered in one way or another.

Hopefully, after a crank re-build, you'll be up and running again.

Regards

Mark
_________________
If it ain't broke, fix it until it is.