Topic: CB350F: two failed top end rebuilds, stock bore and 393cc

[dave500]

yeah as disco says hope its only residual oil in the pipe?trouble is you need a good summer long ride to truly blow all the old crap out,ive assembled 550 motors from old used pistons and barrels swapped around with rough as guts hand honed with wet and dry paper and never had this drama,in fact one goes as well as a proper rebore with pistons etc sized up and bores cut/honed to size and tagged!i used that motor when I broke a gear as a spare motor and left it in for ages even after I fixed the other as it went so well?
don't be scared to work a new engine hard, just don't go nuts and peak rpm every gear at first,imagine in the day new buyers just hankering to try all that performance in their new bike they were promised?

[robvangulik]

Just do some long rides, why would that only be possible in summer?
If you get cold or wet, you didn't put the right gear on!

[markreimer]

Just do some long rides, why would that only be possible in summer?
If you get cold or wet, you didn't put the right gear on!

LOL I'm not sure you understand what the weather is like here. Since last night, an inch of fresh snow has fallen on the road. I live in a small-ish city and there were over 100 accidents on bridges due to ice a few days ago, to the point the police shut most of them down. I'm not afraid of riding in the cold, but I won't ride when it's icy and even the semi's can't stay on the road 

Maybe I'll get lucky and we'll have one last warm spell before it plunges down to -30. I tried riding once on a dry day in that kinda cold. It sucked.

Hopefully its just old oil burning out of the pipe. I don't know if doing 80mph, in the first 5-10 minutes of a new piston/bore is gonna do it any good though? You can't be accused of 'lugging' it I guess 

I also hope it's just old oil, but that doesn't explain the wet plugs then. I think there are just too many things at play here to make any proper conclusion. The rings haven't bed in, the pipe hasn't burned out, who knows. RE: 80mph, that was a small typo. 70mph, I had cold fingers and was on my phone lol. But even so, I kept the RPM in the 6-7K range, no higher. I started it up, let it idle a minute or so, blipping the throttle a few times to see how the engine sounded and reacted. It sounds very smooth. Then rode it around the block 3-4 times. I don't wanna break a ring off the bat, don't worry!

yeah as disco says hope its only residual oil in the pipe?trouble is you need a good summer long ride to truly blow all the old crap out,ive assembled 550 motors from old used pistons and barrels swapped around with rough as guts hand honed with wet and dry paper and never had this drama,in fact one goes as well as a proper rebore with pistons etc sized up and bores cut/honed to size and tagged!i used that motor when I broke a gear as a spare motor and left it in for ages even after I fixed the other as it went so well?
don't be scared to work a new engine hard, just don't go nuts and peak rpm every gear at first,imagine in the day new buyers just hankering to try all that performance in their new bike they were promised?

Isn't it so weird? This bike is cursed it seems. I've done a handful of other SOHC4 top ends (all 750's) and every one of them was just fine. This bike is identical, just smaller. And I've had lots of great advice here, and in person from a few different professional engine builders I'm friends with. Fingers crossed I can bed those rings in properly soon and close the book on this engine.

[Phoenix]

I feel your pain.  Melted a piston and got ready to put in a CR 836 kit.  Went to THE machine shop in town.  He wanted to bore 20 over the pistons.  Nothing else.  Went to another shop.  Explained what I wanted and wrote it down.  Put it together and it leaked and smoked big time.  Just like yours.  Oil on top of pistons and leaking around exhaust spigots and dripping out of exhaust drains.  Took it apart and had the machine shop measure the holes.  He had bored it to twenty ANYWAY.  I said screw it and sent it to Mike.  Only good decision I made.  He decked both sides.  Put in sleeves and bored to the proper spec.  Chamfered the sleeves for easy assembly.  New valves and guides so he could install seals on all of them.  Beautiful job!
Put it back in and still smoked and leaked.  Took it apart and realized I had gapped all three parts of the oil ring not knowing any better.  Fixed that and it is running like a champ.  Although pipes were full of oil, no smoking after I fixed the rings.  As I understand, per Mike and HM, the piston gap is critical and different than most shops think it should be.  Something to do with forged vs cast pistons.  Pretty sure if I sent my rings to Mike along with the other stuff, I would not have had the problem.
As far as CR, n problems.  Asked if rings were marked for up and down.  He said no.  I also re-ordered single ring sets three times because I kept breaking them.  He even put one set together for me.  Good luck going forward.  Re-measure everything.

[Jerry Rxman Griffin aka MuthaF'er]

1 AND 4 makes me first think ignition. Do the plugs look wet or oily?! Smoke = raw gas due to misfiring plus oily exhaust system?

[markreimer]

I feel your pain.  Melted a piston and got ready to put in a CR 836 kit.  Went to THE machine shop in town.  He wanted to bore 20 over the pistons.  Nothing else.  Went to another shop.  Explained what I wanted and wrote it down.  Put it together and it leaked and smoked big time.  Just like yours.  Oil on top of pistons and leaking around exhaust spigots and dripping out of exhaust drains.  Took it apart and had the machine shop measure the holes.  He had bored it to twenty ANYWAY.  I said screw it and sent it to Mike.  Only good decision I made.  He decked both sides.  Put in sleeves and bored to the proper spec.  Chamfered the sleeves for easy assembly.  New valves and guides so he could install seals on all of them.  Beautiful job!
Put it back in and still smoked and leaked.  Took it apart and realized I had gapped all three parts of the oil ring not knowing any better.  Fixed that and it is running like a champ.  Although pipes were full of oil, no smoking after I fixed the rings.  As I understand, per Mike and HM, the piston gap is critical and different than most shops think it should be.  Something to do with forged vs cast pistons.  Pretty sure if I sent my rings to Mike along with the other stuff, I would not have had the problem.
As far as CR, n problems.  Asked if rings were marked for up and down.  He said no.  I also re-ordered single ring sets three times because I kept breaking them.  He even put one set together for me.  Good luck going forward.  Re-measure everything.

Thanks for sharing your experience, sure sounds like what I'm going through.

I had my piston-to-bore clearance done at .0008", definitely 'tight' by most standards.

The machinist raised his eyebrows and questioned my request, but I was sure and he basically said hey man, not what I recommend but it's your engine. He measured it after the work was done, And I took it to an independent machinist to double check. The assembly spec is .0004-.0012, and the wear max is way past that. Mine were bored and honed to .0008", with a slight hourglass shape at .001", so still within spec.

I confirmed my rings were in right side up (all three had 'top' markings), and I didn't need to file them as the gap was just over the minimum spec, other than the compression ring in #3 which needed a tiny pass with a file.

I requested the new guides be set to the clearance in my manual, I think it was .0006 if I remember correctly. Maybe .0004, I'd have to check my notes.


So all the stuff you're talking about having to re-do, I've re-done as well. I don't like hearing that your bike stopped smoking with the old oily exhaust though, I don't understand how that could happen. My machinist reminded me to be aware that I'd get some smoke coming from old oil when I picked up the head too.

I won't be taking this engine apart again this year. I think it's premature to open stuff up again, especially when I KNOW it's all measured within spec, after only one 5 minute ride. There's snow on the ground but first chance I get I'm going to throw a quart of oil in my backpack and ride the piss out of this bike till it either stops smoking or I run out of oil. Then I'll make a call on opening it up, or buckling down and finding a 400F engine.....

[markreimer]

1 AND 4 makes me first think ignition. Do the plugs look wet or oily?! Smoke = raw gas due to misfiring plus oily exhaust system?

I hear ya. I didn't pull 2/3 out, it was past midnight and I just wanted to see if a plug or two was oily and the cause of the smoke, rather than the headers. I gave it the sniff test, didn't smell gassy to me.

[innovativems]

I think you first need to get those rings  broken in with he recent hone job.  Get it good and broke in


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[markreimer]

I think you first need to get those rings  broken in with he recent hone job.  Get it good and broke in


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Agreed!! Might have to get snow tires first lol.




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[innovativems]

Haha.  Time for some drifting!!! Aka flat tracking


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[disco]

Hi Mark,

That looks seriously cold! I’m with you, break out the skidoo instead! Fingers crossed, come Spring, your hard work will pay off.

[juntjoo]

lol, I should have skipped to the end of this novel to find out there was no final scene with the evil boss. Or are we still waiting for a final assessment?

[markreimer]

The jury is still out!

Mother nature forced a suspenseful conclusion to this story... Winter came and a big snowstorm hit while I was wrapping up the build. It's -20C right now, and the bike hasn't left the garage. Good news is that riding season typically starts around mid-March, so I'm 5-6 weeks away from putting some miles on the bike.

Stay tuned....

[grcamna2]

The jury is still out!

Mother nature forced a suspenseful conclusion to this story... Winter came and a big snowstorm hit while I was wrapping up the build. It's -20C right now, and the bike hasn't left the garage. Good news is that riding season typically starts around mid-March, so I'm 5-6 weeks away from putting some miles on the bike.

Stay tuned....

Would be nice if you break it in on a dyno inside where it's warm

[markreimer]

Haha I’d be kicked out after 30 seconds with the amount of smoke burning out the pipe...


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[Smithy]

Cal - the engine smokes all the time, even at idle. It just smokes the worst after engine braking and then rolling on. But there is definitely a steady stream of blue smoke coming out the pipe. I did replace the intake valve guide seals when I rebuilt the engine, everything was OEM/Honda.

During the original rebuild , did you use OEM rings or Chinese copies? OEM rings would have been expensive.
I have CI pistons in my 350F and have tried 3 sets of rings and It still smokes like crazy. Currently trying some break-in oil that is suppose to chemically assist ring sealing but not helping so far.
So was wondering if OEM rings would be any better?

[dave500]

you need a place that bores and hones each bore and mates a piston to it,then check ring gaps,its an expensive thing doing it twice to end up with still a smoker that's just crap.
don't let the motor idle on first run just ride it as you would but don't over rev it,7000 rpm wont hurt it.

[markreimer]

Smithy, I’ve done both. Original pistons with new Honda oem rings- smoked. CI 393cc kit with CI pistons-smoked.

Dave, that’s exactly what I had done. And after it still smoked I took it to a different machine shop to verify clearances and everything checked out. That’s when I replaced the guides with ones with exhaust seals and haven’t had a chance to test.

All I can say at this point is that when I opened the bike up I could tell the rings hadn’t broken in yet. The second control rings still had some black phosphate coating on the outside surface. And the pipe was had oil pooled in the lowest point from massive oil consumption. So until it’s spring and I can ride this long enough to break in the rings and burn out the pipe, the jury is out


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[DMMartin]

Thought I might chime in with some thoughts:

I'm new to the little 4 engine building and have a 350f myself that I am having issues with it smoking after my first top end. However, I have built many automotive engines before. If you look back the wear marks on your rings you posted on page 7. It appears as if the second oil ring may be upside down. I'm not 100% certain but here is what I'm basing this off of:

I did a quick and dirty top end rebuild on my 350f with new ebay rings:

https://www.ebay.com/itm/Honda-CB350F-CB350-Piston-Rings-Set-x-4-sets-1972-1973-1974-Std-Standard-Four/382387216462?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I did a light hone myself, lapped the valves, and replaced all 8 valve seals (mine had 8 when I took it apart) got it all together and it smoked more than ever. I only found oil on 2 and 4 plugs. Pulled it all apart and found very different wear marks on the 2nd ring of those cylinders. Where my coating had worn away left a nice shiny band on the bottom side of the 2nd rings in numbers 1 and 3. In 2 and 4 (the ones that where burning oil) I found that same wear mark on the TOP side of the 2nd ring. From this I decided to flip over the rings in 2 and 4. I'll be putting it together tomorrow and I can let you know what happens. My 2nd ring had no mark for top and no distinguishable scrapper on them so I was unsure on the assembly. Some of your ring pictures seem to show a wear pattern on the top side of the 2nd ring.

One other thing is on #3 I did notice what seems to be a very small amount of oil getting past the exhaust guide with a "wiggle" measurement of .0028" and new seals.

Once again I still haven't gotten mine back together so I can't tell you for sure but hopefully this might be able to help you some. I'm only on top end 2.0 and I've had enough. Can't imagine how you feel after reading this all

[markreimer]

Very curious to hear about your results.

As for my bike, I threw the towel in and sold it about two weeks ago.

I found a young guy with a wanted ad, looking for a 'project Honda CB'. He showed up with his Dad, I gave him the full run-down of the oil burning issue, aftermarket piston kit, all the back and forth I had gone through, and he seemed confident he could figure it out so we settled on a price and off it went.  It was more a 'cut my losses' kind of situation. I definitely lost a lot of money on it in the end, but honestly I think the Cruzin Image kit is 100% at fault here and absolute garbage.

As for your comments about the piston ring orientation, I'm 100% positive they are all put in right-side-up. I actually have photos of every ring on each piston from the last top-end I did, where you can see the markings, so I would KNOW they were in properly if I doubted myself later on. I've got 'em saved somewhere. I never did get to test it after the new guides and valve job, but I suspect i'd have been in the same boat. Those rings are no good in my opinion. Then again maybe I'm wrong and the kid will take it on a break in ride, it'll smoke for 50 miles and then clear up LOL. I hope it does.

I just bought my first house so I can't be dropping $$$$$ on the third or fourth try at an engine rebuild. Instead I've been out riding my K4 happy as a clam with no issues. 

[PeWe]

Good to have more than one bike when one fails. Smoking after a rebuild is not fun, worse than heavy oil leaks.
Time for an oil (smoker) thread.... How oil can enter the combustion chambers when pistons, rings, bores, guides are checked and still smoking.
Smoke after heavy throttle, release it and give throttle again is typical guide seals. I had a car that made the road behind me invisible
I read somewhere about oil press inside the sleeves up to the top passing the head gasket.....

[dave500]

ive noticed a few 350 smoker threads?i haven't looked but were they all the cruising image kits?ive had a david silvers 550 first over size kit in mine for ages and I don't spare it,ive also got a spare motor I used not so long ago for a couple of months and that was only a total tear down and reassemble with all the same re used hard parts but new seals through out,never an issue,for what its worth the the spare motor goes like the rebored one!
its heart breaking to have done a good job being carefull and clean and all the money spent to have inferior results.

[bryanj]

There is a topic on the UK site where a man has a 350 with one smoky bore after rebuild and he is adamant that oil is passing from the oil jet to the bore because of a crap gasket, I have my doubts

[Trevor from Warragul]

The problem is most likely to be the rebore. The pistons on the CB350F are very small. If the cylinder is bored out to the correct clearance, more than one pass with the hone can take it out too far.

I had mine bored out to 2nd oversize & installed OEM pistons & rings. One cylinder oiled up something chronic. In the end, I bought a used block & pistons on standard bore. I installed them with new OEM rings, and the bike now runs fine.

[dave500]

must be some #$%* machinists around?ive had more than a couple of small 75cc 4 stroke singles rebored with no trouble.