Author Topic: 1980 CB125S seized piston (Read 9034 times)

Pielz · « **on:** May 10, 2020, 08:54:34 PM »

I already have the cylinder soaking with PB blaster over night as well as a liberal amount sprayed on the valve springs and guides, but I'm wondering if anyone has any other advice if I decide to try any other methods that have worked well for you. Would carb cleaner soaking in the cylinder be ill-advised, even if I used a lubricant immediately afterwards?

Bonus question, what bolt on this engine can I use to turn the crank by hand? I tried a wrench on the bolt under the timing cover (under the lower left-front case not the points plate bolt) but wussed out when I could feel it tightening slightly. Didn't want to over-torque or strip it. Haven't seen anything in the manual about whether there's a best practice for turning it by hand to try to loosen it.

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grcamna2 · « **Reply #1 on:** May 10, 2020, 08:59:28 PM »

Hi Pielz,
I edited a couple of my posts to include how to do this,that should work.

Pielz · « **Reply #2 on:** May 10, 2020, 08:59:54 PM »

I'll check it out, thank you!

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grcamna2 · « **Reply #3 on:** May 10, 2020, 09:01:01 PM »

Glad to help

HondaMan · « **Reply #4 on:** May 10, 2020, 09:01:36 PM »

If it doesn't come free, don't force it or it WILL break. And, if it is that bad, it won't run, either. The 125 is easy to rebore and rebuild, lots of piston-rings kits still out there as they are one of the most popular bikes in the world. Still!

Did you know: it has the same cam timing, ignition advance, and carb as the 750?

Pielz · « **Reply #5 on:** May 10, 2020, 09:07:00 PM »

Quote from: HondaMan on May 10, 2020, 09:01:36 PM

If it doesn't come free, don't force it or it WILL break. And, if it is that bad, it won't run, either. The 125 is easy to rebore and rebuild, lots of piston-rings kits still out there as they are one of the most popular bikes in the world. Still!

Did you know: it has the same cam timing, ignition advance, and carb as the 750?

Awesome thank you! I didn't know that. I planned to rebuild it regardless and have a full gasket kit in the mail, but I was going to see if I could get it to start up or at least free up in the meantime. I very gingerly rocked it in 1st gear but was nervous about how much force I should use so I just figured I'd let it sit overnight and try again tomorrow. It's been soaking for about 8 hours now. With the crank case full of new oil and the piston full of penetrating fluid. How gentle do I have to be when doing the rocking method?

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Pielz · « **Reply #6 on:** May 10, 2020, 09:08:07 PM »

Quote from: grcamna2 on May 10, 2020, 09:01:01 PM

Glad to help

Your post was helpful, thank you! I had it in 1st and 2nd, not any gear higher. I'll have to try that in the morning. Good news is the front brake unseized while I was at work lol it works properly now! Now if the engine would follow suit haha

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grcamna2 · « **Reply #7 on:** May 10, 2020, 09:09:33 PM »

Quote from: Pielz on May 10, 2020, 09:07:00 PM

Quote from: HondaMan on May 10, 2020, 09:01:36 PM
If it doesn't come free, don't force it or it WILL break. And, if it is that bad, it won't run, either. The 125 is easy to rebore and rebuild, lots of piston-rings kits still out there as they are one of the most popular bikes in the world. Still!

Did you know: it has the same cam timing, ignition advance, and carb as the 750?
Awesome thank you! I didn't know that. I planned to rebuild it regardless and have a full gasket kit in the mail, but I was going to see if I could get it to start up or at least free up in the meantime. I very gingerly rocked it in 1st gear but was nervous about how much force I should use so I just figured I'd let it sit overnight and try again tomorrow. It's been soaking for about 8 hours now. With the crank case full of new oil and the piston full of penetrating fluid. How gentle do I have to be when doing the rocking method?

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Put it in 5th gear

Pielz · « **Reply #8 on:** May 10, 2020, 09:14:15 PM »

Quote from: grcamna2 on May 10, 2020, 09:09:33 PM

Quote from: Pielz on May 10, 2020, 09:07:00 PM
Quote from: HondaMan on May 10, 2020, 09:01:36 PM
If it doesn't come free, don't force it or it WILL break. And, if it is that bad, it won't run, either. The 125 is easy to rebore and rebuild, lots of piston-rings kits still out there as they are one of the most popular bikes in the world. Still!

Did you know: it has the same cam timing, ignition advance, and carb as the 750?
Awesome thank you! I didn't know that. I planned to rebuild it regardless and have a full gasket kit in the mail, but I was going to see if I could get it to start up or at least free up in the meantime. I very gingerly rocked it in 1st gear but was nervous about how much force I should use so I just figured I'd let it sit overnight and try again tomorrow. It's been soaking for about 8 hours now. With the crank case full of new oil and the piston full of penetrating fluid. How gentle do I have to be when doing the rocking method?

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Put it in 5th gear

I saw your edited comment, I'll be sure to do that. I've been fortunate that the 4 vintage bikes I have experience working with so far had unseized engines so I'm not used to this particular set of circumstances

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onepieceatatime · « **Reply #9 on:** May 11, 2020, 03:26:30 AM »

I freed 3 of them up with 50% acetone 50% ATF in the cylinders. 2 of them were 750s, the other was a 400A. being an A (Hondamatic) I could not rock it in gear. It sat for about a month with that stuff in the cylinders, then I used a wrench on the crank to move it. it moved like it was never stuck at that point. The 750s, rocking gently in gear only sat for about a day each. Make sure you put a rag over the spark plug hole! I didn't with one of the 750s and ended up cleaning up ATF from everything it sprayed on over about 30 feet in the garage.

Pielz · « **Reply #10 on:** May 11, 2020, 07:37:55 AM »

That sounds like it'd work pretty well. I'll grab some ATF and acetone when I stop at Wally world in a few mins and if the PB blaster hasn't done anything yet I'll give that a go. Thanks for the tip!

Also I've made that mistake in the past lol. Accidentally shot chemicals all over my car and everything in the vicinity

it's a lesson you only have to learn once, for sure.

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seanbarney41 · « **Reply #11 on:** May 11, 2020, 01:16:34 PM »

rags schmags...real sohcer's go for the distance! I believe Steve-O is the current record holder at 28ft.?

Pielz · « **Reply #12 on:** May 11, 2020, 04:06:10 PM »

Haha sounds like a record!

Also a lil update. Took the clutch cover off to have a look to see if anything is broken or sheared. With a flashlight and moving the kick starter through it's free range, it doesn't appear that the gear on the kick starter spline is able to move.

I've got a little cocktail of liquid in the cylinder composed of a little sea foam, berrymans, and marvel mystery oil. Upon closer inspection, it seems that the exhaust valve is fully open. I'm not sure what the typical valve travel is for this model but I can see the very rim of it when I'm looking through the spark plug hole. Does that sound like a dropped valve?

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Pielz · « **Reply #13 on:** May 11, 2020, 04:23:02 PM »

You can see it there.

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grcamna2 · « **Reply #14 on:** May 11, 2020, 06:44:41 PM »

Have you been working the cycle back and forth in high gear ?

Pielz · « **Reply #15 on:** May 11, 2020, 08:48:54 PM »

Yep! I actually just tore the head and valve cover off. Cam lobes up top are beautiful. Valves are not stuck and seem to seat pretty well. The piston is so caked with carbon and rust though. Pretty gnarly. The kick starter actually functions fine it's the dang piston. Im going to definitely have to hone it.

It's almost midnight here so I'm heading home to get ready for work tomorrow. Tomorrow afternoon I gotta remove the rotor for the alternator using the axle, I heard you can do that since the tool is nowhere to be found online. I accidentally dropped the cam chain down the jug so I have no hope of getting it now without removing the stator

that's fine either way cause I need the crank to move freely so I can tap the piston from the top with a block or something. Don't want the cam chain getting in the way of that.

Once the jug moves independently of the piston I'll be able to evaluate if the gear mechanism and kick starter do actually work properly though. But the kick does move the piston as it should.

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Pielz · « **Reply #16 on:** May 11, 2020, 09:08:55 PM »

Also one more thing I noticed. The cam chain tensioner bar has a small crack in it up at the top where the bolt goes through it that holds it in place up by the head. It's cracked straight through under that lobe. The part seems to be pretty tough to track down from US based sellers online. I'll post a pic tomorrow and hopefully one of you can tell me if I can still run it or gotta replace it. Guide bar has some wear but doesn't look awful.

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grcamna2 · « **Reply #17 on:** May 11, 2020, 09:41:17 PM »

You did jump right into it ! Well you should be able 'fish' that cam chain out of there with a hooked length of mechanics wire,it has never failed me that way.Cam chains are easy-fishing.. real fish are tough

Pielz · « **Reply #18 on:** May 11, 2020, 10:17:36 PM »

That's the thing, in our attempts to fish it out, it was sticking to the magnets in the rotor and got all wrapped up around it.

I really bungled that.

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grcamna2 · « **Reply #19 on:** May 11, 2020, 11:29:06 PM »

Quote from: Pielz on May 11, 2020, 10:17:36 PM

That's the thing, in our attempts to fish it out, it was sticking to the magnets in the rotor and got all wrapped up around it. I really bungled that.

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Got it

When you remove the rotor,be careful Not to whack the end of the remover bolt(axle)with a hammer:that can change the balance of the crankshaft..
I think an electric 1/2" dr. impact gun will be better to turn the remover bolt.

Pielz · « **Reply #20 on:** May 12, 2020, 06:41:13 AM »

Thanks for the advice! I'll do that. Fortunately I just bought a new bosch impact!

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onepieceatatime · « **Reply #21 on:** May 12, 2020, 07:19:24 AM »

Quote from: Pielz on May 11, 2020, 08:48:54 PM

...so I can tap the piston from the top with a block or something.

I did that on a CB650 engine. It was not easy , but later I read that if you put some Kerosene or diesel in the cylinder and light it, then when that all burns off try knocking the piston out, it will usually go easier. I imagine heating the whole cylinder assembly with a propane or Mapp torch would work as well and look a little more sane and rational.

Pielz · « **Reply #22 on:** May 12, 2020, 08:11:01 AM »

Oh good idea! Warm everything up and maybe get some expansion going. I'll do that before I start hammering away. Thanks a ton!

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Pielz · « **Reply #23 on:** May 12, 2020, 08:12:42 AM »

Btw, when I took the valve cover off and leaned the camshaft over to take the sprocket off, this fell out on to the bench. It looks similar to the pins in the cam chain but I checked the whole thing and there are no missing pins. It kinda looks like an indexing pin or detent key for something but I couldn't find anything it appeared to go to.

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grcamna2 · « **Reply #24 on:** May 12, 2020, 09:52:12 AM »

That's the small pin that fit's into the camshaft to 'key' the points advance mech at the correct position.

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Author Topic: 1980 CB125S seized piston (Read 9034 times)