CB125T Engine woes

[GeneralXcepti...]

Hi all.

I'm rebuilding an '83 CB125 TD Super Dream for a relative. Part hobby / part ambition to restore 'something' before I'm too old to turn a ratchet any longer

I've already rebuilt the engine top and bottom end because it was leaking oil, burning oil and a few other issues. I re-lapped the valves, replaced all the oil / valve seals etc.

Good news is I've fixed all those issues except for the burning oil. I know I should have done a compression test before I stripped it the first time but the two (!!) compression testers I was sent from eBay turned out to be total junk . I couldn't be arsed to wait any longer and stripped it down.

Anyway, I've got a decent compression tester now (thank you Gunson) and the readings are: Left hand 98psi dry + 148psi wet, Right hand 100psi dry + 165psi wet. I guess this is pointing to the rings / cylinders being knackered.

Couple of questions:

1. Is it worth while doing a leak down test? I don't have the kit but have access to a compressor, I realise I have to put the cylinder in TDC but what PSI should I use to test if I should bother at all?

2. When I last took the engine apart the cylinders appeared ok - no deep gouges etc. Is it usually ok to replace the rings without honing / boring the cylinders or should I get a shop to take a look at the cylinders for me?

Just to add, this is the first engine strip down I've done so I could have buggered something up!

Any advice very much appreciated!

Gen

[GeneralXcepti...]

Update:

Decided against trying to do a leak down test because I already know that the rings / piston / cylinder are fubar and that needs sorting whatever.

Stripped the head and this is what the pistons looked like:
https://img299.imageshack.us/img299/6653/pistonsabove.jpg

The LHS cylinder was 'working' better than the other cylinder. The RHS cylinder looks to be in better condition but it's the one that was smoking when it was running!

Similar story with the cylinder head
https://img31.imageshack.us/img31/1088/undersidecylinderhead.jpg

I can get a 0.1mm gauge between the piston and cylinder which is the absolute max I should be able to (taken from the Haynes manual). See the stamp on the top - I thought that meant that they were oversized?
https://img532.imageshack.us/img532/1696/01feelercylinder.jpg

So I took the barrels off, took the LHS piston out and measured the pistons and piston ring end gaps.

https://img34.imageshack.us/img34/4452/pistonmeasure.jpg

I thought that these were oversized pistons because of the stamp on the top but it doesn't look like it from my measurements. 44mm standard (give or take a gnats) but measuring it around the gudgeon pin I get it to just under 44mm.

Haynes manual says that the wear limit is 43.8mm so they are right on the edge of needing replacement - which backs up the 0.1 feeler gauge measurement I took above.

Piston rings are also worn. First (top) compression ring when placed in the cylinder and pushed into place with the piston shows the end gap to be 0.7mm, the wear limit is 0.5mm. The 2nd ring gap is 0.45mm so it's almost on the limit too.

Good news, the cylinders look to be unmarked.
https://img524.imageshack.us/img524/309/insidecylinder.jpg
https://img13.imageshack.us/img13/2764/insidecylinder2.jpg

Couple of questions:
1. Can I get away with just changing the rings? I can get a genuine Honda set for £17ea from David Silver (https://www.davidsilverspares.co.uk/parts/part_1204/)
2. Or should I change the pistons too because they are on the wear limit? £35ea from Wemoto (https://www.wemoto.com/bikes/Honda/CB_125_TDC_TDE_Superdream/82-87/picture/Piston_Kit_0.00_Standard/). Prob not genuine Honda.

As always, thanks for any help with this.

Gen

[Kickstart]

Hi

Personally, once at that stage and with the pistons borderline (and the wrong side of that border) I would replace them. However you probably need to measure the barrel to find out whether you can get away with stock size pistons or if you need oversize ones.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[GeneralXcepti...]

Thanks for your reply Keith.

I think that seeing as I've got it apart (again) it makes sense to replace them. I've measured the bores and they are a standard size, so that's what I've ordered.

I'll let you know how it goes.

Cheers

Steve