B120 Crank Seal

[othen]

Hi,

This is my first post. I acquired a 40 year old B120 a few weeks ago. It is a barn find, but I have the original V5 (one owner in 40 years, but it has been laid up since 1986, the tax disk from that year is still fitted).

The bike was a bit rough when I got it: surface rust almost everywhere, the ignition switch compromised, in need of some welding on the swinging arm. It rolled but was far from a runner (I'll post some photos later when I work out how).
I've made a patch for the swinging arm and welded it in, found a replacement ignition switch (from Thailand, it only cost $15 and came in less than 2 weeks), rewired the bike (some things even work, but there are lots of bad connections and earths to find) and got it going...

Having got it running a few days ago my problem now is that the drive side crank seal looks like it has failed. The bike runs but produces lots of smoke and has liquid oil coming out of the exhaust. I'm pretty sure it is consuming the gearbox oil, so I've drained that and there was quite a bit less than the 800cc I put in.

I've found a seal on Fleabay (for just £4.70 delivered, should be here in a few days). I last changed a crank seal when I was in my teens (I think it must have been either a Villiers or an Ossa) - I don't remember it being all that challenging, but I wonder whether there is anyone who has done the job on a B120 (or similar), or perhaps has a workshop manual?

This has been an interesting project, my aim is to put the bike back on the road for minimal cost and run it around locally as a 'survivor' restoration.

Alan

[Craig380]

Hi there,

it isn't NAD408P, by any chance? That was my dad's old B120

Here's a link to the B100 shop manual, which will hopefully help .... the engine didn't change THAT much over the years, from what I remember ...

http://www.oldjapanesebikes.com/mraxl_G ... /index.php

[othen]

Hi there,

it isn't NAD408P, by any chance? That was my dad's old B120

Here's a link to the B100 shop manual, which will hopefully help .... the engine didn't change THAT much over the years, from what I remember ...

http://www.oldjapanesebikes.com/mraxl_G ... /index.php

Hi,
Many thanks, the manual is superb, just what I was looking for. Now I can see how to take the oil pump and clutch off without damaging anything.
The bike is not your dad's old one, it is SYB 864R. I've tried uploading a photo, but no success so far.
Best wishes,
Alan

[othen]

The right hand side of the engine came apart very easily, but there was some suspicious debris in the bottom of the transmission case. When I got the crank pinion off the extent of the bad news was revealed. The crank seal was in several parts, and with it were various bits of the main bearing cage. The top quarter of the main bearing it is missing (I'd upload a photo if the site allowed me). It is a little surprising that the bike ran in that state - I suspect it was the reason it was retired 30 years ago.

So, now I have to decide whether it is worth splitting the cases to change the main bearing, or perhaps I part out the bike? I suppose changing the bearing would not be so difficult, but I'll think about it for a day or two. Any views on whether it would be a worthwhile job or not would be appreciated.

[Craig380]

That's a shame. With that kind of problem, the engine would probably need a complete refresh of the crank, bore etc. Hopefully the crankcase isn't damaged around the main bearing, but if it is, it'll need rebuilding with weld or a pair of replacement cases.

At a guess, a crank rebuild by a specialist together with a rebore & piston kit would be £250 - £400 depending on whether the rod / crankpin / webs can be reused. This cost could increase if the cases need repairing, or you find any additional problems in the gearbox, etc. And although it's tempting to try and find a secondhand engine on eBay, remember that you've already got one of those .... you might end up with two engines that need fixing.

It really depends if you bought the bike to restore it to good running order no matter what problems you encountered, or if it was just to tinker with and see what was possible. B120s will never be valuable or sought-after by the classic bike 'market' but like the Honda C50 and Yam YB100, they really do represent an era of biking.

Like Kenny Rogers said, you've got to know when to hold 'em, know when to fold 'em, know when to walk away, and know when to run

[othen]

You are right about what to do.

I've pretty well decided to fix the bike as long as I don't find any real horrors when I pull the crankcases apart. I ordered two new bearings (only about a fiver, delivered) and a gasket set (only £8) and I'll check the internals over as I go. I'm not going to throw money at this (you rightly say, a B100/120 is never going to be worth anything much) but I'd like to get it going okay, even if we just ride it round the private roads at our place in Norfolk.

Once I've got the crankcases apart I'll decide what is worth saving. Left of arc is that I part it out and get my money back (probably plus some - I suppose I might also be able to sell the registration), right of arc is that it just needs bearings and seals (and maybe a standard piston and rings - only about £20), so I put it together and get it going.

It has been a fun little project and I'd still like to save the bike if it is practical.

Alan

[Craig380]

Good luck with it, they are tough old engines so hopefully it may not need major surgery.

[othen]

Many thanks.
The engine/gearbox is coming apart quite easily, apart from those pesky machine screws that hold the two halves together. I'm told they are JIS rather than Phillips heads, so I'll source a proper bit for the impact driver before I try to remove them.
I can't see any debris from the bearing cage in the crankcase, so I'm hoping damage will be limited to just the right side bearing and seal (fingers crossed)...

[othen]

I have re-assembled the bottom end, I didn't find any more damage when I changed the right side bearing. I had a look at the left side and decided to leave it alone as everything seemed okay, and I might have done more damage by pulling it apart.
Here is a photo of some Hard Rock Cafe drumsticks being put to good use...