CB125T crankshaft

[ajb235]

I stripped the engine out of my 1981 CB125T today, to try and find out why it is running badly, a slight tapping noise, and some smoke from the exhausts.
With the head and barrels off, I find 3-4 thou up and down movement on the big ends, when I grab hold of the con rods, and some "wobble" on the small end bearings.
So I am looking for a good used crank with con rods. OR have my big end bearings replaced, with new conrod and bearing assemblies.
I'm favouring the latter. Are kits available for this bike?

I've yet to strip the head and look at the valve guides.

I put a new 142cc barrel and pistons in 1000 miles ago. They seem to be unworn.

[Robby]

You might find aftermarket parts, probably best talking to David Silver Spares. Anything still on the shelf will be old stock, this motor has been out of production for a long time.

Or just take it to a crank shop and ask to have it fixed. Assuming a pressed together crank, it needs a professional with the right tools to do anything to it.

Of course, it is worth finding out if it is out of spec. A bit of smoke and tapping points to top end trouble. Bottom ends tend to rumble and knock.

[Hong Kong Phooey]

It'd be unlikely that the crank would be worn, most likely any wear would be to the shell bearing surfaces. There's not a lot of oil on smaller capacity bikes, and the filtration system is usually crap. Associated owners usually don't stick to the low mileage interval oil changes, and barely bother to keep it topped up. As well as having to rev the tits off it to go anywhere, and low manufacture costs (lesser quality components) this is a perfect storm that makes smaller capacity bikes less reliable.

I don't know the acceptable tolerances on this bike, but keep in mind you're testing it without oil being pumped round, the tolerances on a running engine big end will tighten due to floating on a film of oil. You could be feeling a slight bit of play combined in big and small ends, but how are you measuring shell clearance? If it is indeed 0.004" then that most likely is out of spec even if that's total movement of both bearings combined (up/down).

As for smoke, if it is oil smoke then rings and as you already know the stem seals are usual culprits. I think once you've got it stripped that's the hardest bit done, if you know what bearing surface measurement you need then it might not be that expensive to order. I take it the plug is black?

It might be gaskets and other sealing parts that you struggle to source.
____________________
'81 CG125, '97 FZS600 : '99 CBR600F4, '09 KTM RC8

[ajb235]

Further strip down today.
The exhaust valve guides are worn, the valves have more "slop" than they should have. Easy to source and replace.
The small end pins in the piston have 1.5 thousandths inch wear. This is disappointing after new pistons fitted at the last rebuild 1000 miles ago.
However, Im now paying the price for fitting a cheap Chinese 142cc barrel and pistons kit. The pin materials spec. and finish must be well below OEM spec. I don't think I've ever seen a worn gudgeon pin.
Piston top rings have a gap in the bore of 0.016 thou.The bore has no perceptible wear. I haven't checked the standard spec, but it seems excessive ring clearance for a 47 mm bore.
With the pistons off , the big end wear seems a lot less, maybe 1.5 thou on one side, less than this on the other. Yesterday I was feeling the combined big end wear and the gudgeon pin wear.
I think I'll leave the big ends for now, but I will check the manual for wear limits. I'd like to stay with 142cc, so I'll be looking for some better 47mm pistons, or even just some decent pins, if I can find out what Honda bike they were based on.

[Fizzer Thou]

[ajb235]

I'm fitting new inlet and exhaust guides to my CB125T. I've bought the guides from CMS NL but realise that they need reaming out after fitting to the correct size. Then the valve seats will have be recut
This is now a job for a specialist machine shop.
Is there a good machine shop or engine reconditioned in the UK that could do this? Re O mendacious welcome.

[ajb235]

I'm fitting new inlet and exhaust guides to my CB125T. I've bought the guides from CMS NL but realise that they need reaming out after fitting to the correct size. Then the valve seats will have be recut
This is now a job for a specialist machine shop.
Is there a good machine shop or engine reconditioned in the UK that could do this? Re O mendacious welcome.