Hi all,

I've been reading this forum for many yrs but this is the first time i register an account and post.
As u all know, PGM4 crankshaft is prompt to have the center seal broken.
And we have to purchase a whole new crankshaft just becos it do not sell the center seal alone.
i read abt the post on substitute seal by Koyo bearing.
Do any1 knows wher can i purchase this Koyo bearing for my MC28?
Any gd/bad feedback on this Koyo bearing?

thx for answering my question!!
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HONDA NSR250 PGM4 MC28

Hello wahzai and welcome to the forum.

First things first, please can I ask you not to use abbreviated TXT SPK in posts.

Second, the MC21/MC28 centre seal is available as a separate part. You should buy a couple as it's easy to break them, stretching them over the crank webs and con-rods, but they can be replaced.

The bearings come up for sale from time-to-time, but there are no long term reliable sources for them. Just keep an eye out on the forum for them.
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Andy.
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wahzai wrote:Hi all,

I've been reading this forum for many yrs but this is the first time i register an account and post.
As u all know, PGM4 crankshaft is prompt to have the center seal broken.
And we have to purchase a whole new crankshaft just becos it do not sell the center seal alone.
i read abt the post on substitute seal by Koyo bearing.
Do any1 knows wher can i purchase this Koyo bearing for my MC28?
Any gd/bad feedback on this Koyo bearing?

thx for answering my question!!

Good day all. I have been pondering this for a while. What would be the first symptom of a partially failed center seal. Is it possible for the engine to even run ( however badly ), with a totally failed center seal. Thanks

poor starting is a good sign as the crank cases just dont pressurise as they should, the centre bearing and seal were shot on the crank i've just taken out, it was a sod to start, but once running it was ok, allthough performace was not too sharp.

the seal going was just a symptom of the bearing failing though, first one i've ever had

If properly looked after, the seal will go long before the bearing(s). As ricey3 says though, hard starting is often a classic symptom.

The centre seal [that commonly fails] on an MC21/28 is basically just a big 60mm O-ring.

Edited for increased clarity.
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Andy.
NSR-WORLD.COM

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Last edited by Andy on Tue Feb 10, 2009 8:55 pm; edited 1 time in total

thanks you guys for the prompt reply.
So can the Koyo Bearing solve the problem on the centre seal ?
i saw it selling at 1 of the china "ebay".
But i do not know how reliable that is.
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HONDA NSR250 PGM4 MC28

My crank is rebuilt with genuine Koyo's sourced from Australia, and I've had several cranks rebuilt for other forum members (again with Australian bearings, and some from Greece). Although bearings from China may/should be OK, I have serious reservations in trusting what's "genuine" from that part of the world!

Someone has to be the first to try them, but it certainly won't be me, and probably won't be anyone I know personally either!

Replacing bearings won't [necessarily] fix [symptoms often mistaken as] a broken centre seal! As I already said, the centre seal [that seems to cause most problems] on an MC21/28 is an O-ring. It runs around the outside of the centre bearings, and can be replaced without splitting the crank.

The later centre bearings do have integrated seals to stop fluid transfer from one side of the bearing to the other, but I don't expect these fail like the O-ring.

Edited for increased clarity.
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Andy.
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Last edited by Andy on Tue Feb 10, 2009 8:57 pm; edited 1 time in total

So would I be correct in assuming that the center seal is a labryinth type,(MC21/28) and that the 60mm oring seals the labryinth housing to the crankcase ?

The outer o ring on the mc21/28 is not the main seal. It only seals the outer side of the race to the crank cases.

The center bearing its self has a seal built into it which seals on a lip on the right hand inner crank web.

Replacing the O ring on the center bearing does not cure a leaking center seal.
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If I have to take the carbs off once more...

No, but the O-ring is the part that fails most commonly it seems. The main seals usually seem to last equally as long as the bearings, looing at the bottom-ends/cranks that I've had experience with.

The outers seem to fail more often than the centres I think, on all models, but the O-ring seems a weak point.
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Andy.
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I think must of the outer o rings fail as people don't lube them when putting the crank into the cases and they are stretched or pinched on re assembly.
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If I have to take the carbs off once more...

I see where my previous post is unclear, and where the statement is ambiguous.

I still think however that O-ring or outer seal failure on the MC21/28 is more common than centre seal failure, and would bet the traditional 50p that more often than not a new O-ring will fix a leak across the crankcase in the later motors. The MC16/18 certainly doesn't seem to suffer the problem anywhere near as much as the MC21/28. I know the (main) centre seal arrangement is quite different between the two, but I've (personally) yet to split an MC21/28 motor that's been together for any length of time where the O-ring's not in poor shape.
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Andy.
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I got it now. I have yet to see inside one of these engines, other than the parts diagrams, which show no internal details. Thanks for the clarification.

Hopefully the edits make my thoughts a little clearer.

Thanks for the prompt to be a bit more accurate Steve!
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Andy.
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