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Discussion Starter #1 (Edited)
This past March I took a (mostly) great trip around the Pacific Northwest with my CB450 K3. She rode like a dream but I made the grave mistake of not checking my oil every fuel fill up. Near Mount Shasta my engine started smoking and I realized I had no oil in the crankcase. I put oil in but I think it was too late. Ran rough from there on out, but I made it home...

Yesterday I finally got around to tuning the bike up..... tons of metal pieces filling the little recession in oil plug. Like I mean shavings that I could pick up and that were hard (no pictures, sorry... my heart was beating too fast haha). Checking the valves, the cams were a bit scraped up too.

150psi on the right cylinder but on the left 100 psi coupled with smoky exhaust.

Does this sound like an engine rebuild? Do I just need new rings on the pistons or do I need to completely re-bore the cylinder?

Thanks for any help with my injured baby.
 

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Sounds like you need to take it apart and see before we’ll know for sure. Technically speaking, I would call a piston ring job an engine rebuild :D I wouldn’t ride it any further until you’ve inspected everything though. My thought is that it’s less likely a cylinder issue and more likely a head issue, with the cam damage.

I’d be looking for:
- Damage to cam bearing surfaces, cam faces, and cam follower surfaces
- Damage to any cam rollers
- Piston wall scoring

If it was totally dry, I’d probably crack the cases while you’re in there. Crank bearing damage is a possibility.

Sorry to hear it all though. Don’t panic, you’ll get her going again! Delta Cams in Tacoma does a great job on resurfacing. Last time I paid like $60 for each cam and $9 per follower, and they came out beautifully.
 

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ugh! sorry to hear this. Yes, you cannot really get away with running it as is. I wouldn't. Motor has to come out and be inspected for damage. Pistons could be scratched and inner cylinder wall as well. This happened to me last season but it turned out ok. I shut 'er down and an ex-pal took care of it with a backup cylinder head but luckily pistons & rings were measured and fine. Sometimes those are the lucky breaks!
 

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Absolutely agree with spirograph, it needs to come apart. The low compression and smoke are likely from galling of the piston on the weak cylinder. The low oil presence causes high friction, making the piston overheat and transfer aluminum to the cylinder wall, usually on the thrust faces of the piston, and will likely have to be bored over to clean up. In cases of light galling, it can be honed and sometimes even the piston can be cleaned up, but when it's run that way for a period of time and under load with minimal oiling in play, it's usually bad enough that it requires parts replaced and overbore. The aluminum transferred to the cylinder wall keeps the rings from sealing, which adds to the oil smoke. The proper way to do it is to bore both cylinders and replace both pistons, rings and wristpins, and the small end of the rod on the affected cylinder may need cleaned up as well since the lack of oil splash allows the wristpin surface of the rod to overheat too. The wear on the cams and followers are the lesser issues here, as they can be cleaned up but the cam bearing caps should be checked as well since they live on oil flow through the cams and can be oil-starved easily. The left side exhaust cam bearing, if worn to a sloppy fit on the cam bearing journal from lack of oil, will affect your ignition timing if you're running points. Here's an example of what can happen with low oil under load - it can be ugly, this one was

http://www.hondatwins.net/forums/8-pictures-videos/45184-73-cb450.html#post449377
 

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Discussion Starter #5
Thank you all for your helpful feedback. Even though I know something serious has gone wrong, definitely puts me a little more at ease to know that others have had this problem before and that it can be fixed.

Thank you ancientdad for the link to that post. Read through it and looks like this might be possible. Will have to wait until I can tear the engine down and start figuring exactly what happened...
 

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Thank you all for your helpful feedback. Even though I know something serious has gone wrong, definitely puts me a little more at ease to know that others have had this problem before and that it can be fixed.

Thank you ancientdad for the link to that post. Read through it and looks like this might be possible. Will have to wait until I can tear the engine down and start figuring exactly what happened...
You're welcome, of course - but the important lesson here is that these air-cooled engines do consume a little oil even when maintained and operated properly, and under load of riding in the mountains even more so. Gotta check the oil regularly on road trips because their capacity is small enough that only a little low can lead to bad things quickly.
 

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Discussion Starter #7
Ok, so after a long hiatus on this project while getting my partner's CL175 road-worthy, I'm tackling my own bike!

I pulled off the cylinder head and cylinder just now.

Definitely some damage. I'll lay it all out as best I understand it now:

Aside from carbon deposits, all the valves save one look good.
All together:
overall valves.JPG

Left side:
Left view valves.JPG

But the right exhaust valve... looks like some chunks out of it.
Right view valves.JPG
Now, the right cylinder had good compression, and so is this an issue? It isn't on the sealing side of the valve, so can I consider this to be purely cosmetic?

At first, I thought the cylinder wall looked good, but then I noticed some slight scoring on both sides. Also where the piston runs is a LOT brighter. Could that be the aluminum from the piston transferred to the cylinder wall that you were talking about Tom?

Right cylinder:
Right cylinder.jpg

Left cylinder:
left cylinder.jpg
 

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Discussion Starter #8
Now the pistons are both a wee-bit rough.
In lieu of a video carefully looking at them from all dimensions here are a few angles:

Both pistons looking from rear of engine:
both pistons rear view.jpg

Right piston:
right piston.jpg

Right piston rear of skirt:
right piston rear view of skirt.jpg

Right piston front of skirt:
right piston front view of skirt.jpg

Left piston - definitely has some damage:
note: this was the side that had low (100 psi) compression. I should add that the upper piston ring is sort of stuck in place...like it has expanded and is too big for its groove on the piston...
left piston.jpg

Left piston rear of skirt:
left piston, rear view of skirt.jpg

Left piston front of skirt:
left piston front view of skirt.jpg

I'm posting all these pictures with the hope that they'll give a good idea of what I'm (we're) up against.

The question that I have is: if I bring the cylinder and pistons into shop to get measured, and they are in spec, could I get through this with some new rings and a hone?
 

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I would not try to reuse that left piston. It is missing significant chunks below the oil ring. I would get the cylinders honed to see if you can stick with new standard pistons and rings, but it looks like an overbore is more likely. You're in for new pistons and rings in any case, and the overbore won't cost that much. My shop charges $40 a cylinder, and I imagine that it should be similar where you are.
 

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Discussion Starter #10
pemdoc65, thanks for the input!

I was thinking it would be safest to go with new pistons and rings, but part of me wants to keep that right piston. Probably best to change it while engine is out though, huh?

I wish machining prices were that cheap here! I just had my CL175 cylinder rebored for $158 at the local shop... just about double what your shop charges. Regardless, I'm happy to spend that money to get a working bike.

I'll jot down the specs and bring my cylinder into the shop tomorrow. I guess they only need to know that the bore should be 70 mm...?
 

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Definitely new pistons and bore after seeing the galling on the left piston, but that exhaust valve needs to be looked at out of the head - it reminds me of a burned valve but doesn't have quite the usual marks on it you might expect to see in that case. Pictures do not always give the view you get in person... I'd pull all the valves and lap them in at the very least, possibly replace tat one exhaust based on inspection. the left piston reminds me of it being run for an extended amount of time with maybe a slightly low oil level, we've seen far worse cases of meltdown from 70+ mph highway runs of long distance as the oil level slowly diminishes
 

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Discussion Starter #13
Tom - very good. I'll pull the valves to inspect. Is lapping something I can do myself? Or better to leave to a machinist to do that for me?
I'm still going to run down to my local machinist with cylinder and see what they think in person.

Bill, I'm assuming you are suggesting stripping the bottom end too,... I could, but the bike did run for about ten hours with some oil after this damage happened (i was deep in a roadtrip). From that experience I under assumption that oil passages are clear...
 

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You can certainly lap the valves yourself, it's a simple process and when done correctly - with a lapping tool and lapping compound, and the valve turned back and forth and not rotated one direction fast, like with a drill as I've seen many do - it's pretty much foolproof. I wouldn't let someone cut the seats unless they really need it, as it can be overdone by someone not familiar with these engines and it usually isn't necessary in the average situation, and it can also lead to less than minimum clearance for valve adjustment once re-assembled since the valves end up deeper in the head, taking up some of the adjustment range. Getting the cylinders looked at, absolutely - sometimes galling can be honed out of a cylinder, but in many cases it takes too much out of the cylinder and it goes deep into the allowable tolerances or beyond
 

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Discussion Starter #15
Just brought cylinders into Baisley's High performance, my trusted motorcycle machine shop in Portland. They are in their third generation of Machining here in Portland and did a fantastic job with my CL175 rebore.
Dan confirmed that I would indeed be best off if I just go oversize. That I could try just a hone but would be risking it because my cylinder probably isn't perfectly straight right now. He measured the size of both pistons and the left one was shrunk by almost two tenths of a millimeter.

So my plan now is to order .5 oversize pistons from Silver Spares, .5 bore, dissassemble the valves and lap.

Will keep y'all abreast of developments!
 

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I recently had my cylinders in a ‘74 CB450 rebored .50 over and used a piston kit from Scrambler Cycle. The match to the original piston was identical from all measurements I took. The kit comes with everything you need. Pistons, wrist pins, clips, and rings for less than $150 shipped. My machinist saw no problems with them and felt for a street bike they should be just fine. I do not have too many miles on them yet but so far I have had no issues. Something to consider to save a ton of money.
 

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Discussion Starter #17
Thanks for the tip! I ended up buying the 450 piston kit (with a .5 bore) from David Silver Spares. Without shipping it costs $112. I bought a fresh camchain link and new chrome mirrors and all together cost $163 shipped. So still affordable!
 

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Thanks for the tip! I ended up buying the 450 piston kit (with a .5 bore) from David Silver Spares. Without shipping it costs $112. I bought a fresh camchain link and new chrome mirrors and all together cost $163 shipped. So still affordable!
Oh wow, I did not realize DSS sold the aftermarket kits. I just assumed they would be OEM and therefore pricey. Good buy for you! I wonder if those are from the same supplier as the ones sold by Scrambler Cycle? Either way good deal and we are both lucky in the sense that a little over $100 gets everything needed to put new pistons in the bike. That’s dirt cheap IMO. Good luck with your rebuild.
 

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Discussion Starter #20
Moving forward on the CB450!

Got my freshly rebored cylinders in from the machinist. Received Piston kit in the mail from Silver Spares.

Spent some time this evening getting the really bad valve out. Here's what it and its seat look like:

IMG_2002.jpg
IMG_2003.jpg
IMG_2004.jpg

I think I'm going to go ahead and buy a replacement valve.... This looks like a good deal, from back in the day when a replacement valve was 12.95! What a dream :)

Also, cams and cam followers are pretty scraped up. Here is a picture of follower, cam surfaces don't look any better.

IMG_1999.jpg
IMG_2001.jpg

Are these surfaces salvageable? I suppose my machinist might be able to tell me that?
 
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