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

[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.

[DMMartin]

Got mine together and running last night. Flipping over the rings fixed the smoking issue. Now #1 has no compression and its blowing out the carburetor. I had some cam chain problems last time around and I'm suspecting a bent valve. Anyone know of somewhere to buy just a head and base gasket without the whole kit?

On a side note I have also heard many things about smoky 350f's and my best guess is that the specs are so tight and the bore is so small that everything "feels" okay or maybe the technique when using a feeler gauge or Mic is off just enough to cause issues

[HondaMan]

The 350F (and its larger cousin 400F) are engines that must be built just right. The piston clearances are 0.0004"-0.0008", although some later manuals have thrown in the 750-sized specs of 0.0008"-0.0012", which will usually cause oil weep past the lower 2 rings. There are O-rings underneath the valve guides, too, and if new ones are not installed with the new guides (when this is done) the engine sucks oil in past the guides.

The bores must be tight, and straight: the pistons must be near perfect. If not, the engine loses efficiency quickly, and this results in smoke from the slow cylinder in the bunch. Vacuum balancing the carbs is a MUST in these little engines, or they run unbalanced and pull oil past the rings of the leanest one.

The 350F and its brother the 250F are engines that require every detail be right! They are like little sewing machines.

[Cb750 Racer]

The 350F (and its larger cousin 400F) are engines that must be built just right. The piston clearances are 0.0004"-0.0008", although some later manuals have thrown in the 750-sized specs of 0.0008"-0.0012", which will usually cause oil weep past the lower 2 rings. There are O-rings underneath the valve guides, too, and if new ones are not installed with the new guides (when this is done) the engine sucks oil in past the guides.

The bores must be tight, and straight: the pistons must be near perfect. If not, the engine loses efficiency quickly, and this results in smoke from the slow cylinder in the bunch. Vacuum balancing the carbs is a MUST in these little engines, or they run unbalanced and pull oil past the rings of the leanest one.

The 350F and its brother the 250F are engines that require every detail be right! They are like little sewing machines.

+1 on what Hondaman said. My guess is a ring or two are upside down. Possibly the valve guide seal o ring as previously mentioned.

[hoodellyhoo]

I was following this thread and hoping for a positive resolution that I could use as a guide for my own 350F top end rebuild but now I’m even more nervous than before that I won’t get it right. I have a NOS set of standard pistons and rings that was made by one of the OEM’s (the name escapes me right now), so I’m not worried about dealing with the cheap eBay rings not sealing. But now I have to hope that a light honing won’t enlarge the cylinders too much to use the pistons I have. I had no intention of touching the valve guides (other than replacing the seals) but now I’m supposed to pop the guides out to replace an oring? Sheesh... I wish I would’ve just let the motor together and lived with the oil leakage at the head instead of opening this can of worms.


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[Smithy]

Hello Chaps, let me tell you a story. I rebuilt a CB350F with an oversize CI piton kit off ebay. It smoked like crazy. Valve guide were mint with new seals and rings were installed correctly. Guessing the re-bore may have been to finely honed I pulled it apart and used my own hone and checked bore to piston clearance and was within tolerance. With new rings it still smoked heaps. Pulled apart again, re-hone new rings and tried some 10W40 break in oil to help the rings bed in. It smoked so bad I got black flagged  Out of frustration I disassembled a spare motor used the stock barrel which was worn and had a score from a broken piston ring, fitted some cheap rings off ebay (not CI) and NO SMOKE. My conclusion is the rings used by CI for the 350F kits are no good. The bike is in the avatar, you can see the smoke coming from the four pipes.

[markreimer]

What rings did you use?? Please share details for the next people who find this thread


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[crazypj]

Assemble cylinder 'dry' with some oil on piston skirts only.
 Oiling bore will lead to rings glazing over, particularly if you 'take it easy' during break in.
 I've built motors this way for years, even putting them on dyno @11,000rpm after 1~2 start up's. You need to hold around 2~3,000rpm so crank flings oil everywhere it needs to go directly after starting and keep it there on a constant throttle until cylinder block/head are on the hot side of warm (probably around 160F?) I also use cheapest oil I can get old of as it's coming back out after first start and 'proper' oil going in instead for 500 miles

Just had a look at pics on Pg7. 3rd pic wear pattern shows ring is upside down as previously noted.
You did check end gaps?
One thing that isn't in service manual (any Honda service manual AFAIK?) but is very handy to know is 'free end gap', basically ring end gap when they are on the bench. It should be the same for all the top,middle bottom sets though not same for top,middle bottom on piston (if you can make sense of that? top runs 'hotter' than middle or bottom) It's the best indication of tension ring will give when fitted to bore and also a good check if stripping a motor to change gaskets, overheated or 'set' rings will have smaller 'free end gap'

[Smithy]

If you have used the 393 CI piston kit you could try buying a set of C90 50mm ring to see if they have similar dimensions. OR buy some CB400F pistons/rings, they have a three piece oil ring and are 51mm diameter. Some people have used them, TTR400.

[dave500]

I don't get this dry ring install thing?,not only do engines need oil on the bores the excess of which will be wiped away as soon as the motor fires but also oil in the grooves,it allows the rings to rotate and cushions the slamming action of the pistons sudden change of direction,try assembling bearings and rocker gear dry,just lube the lot.

[crazypj]

With the second set of rings I installed them dry and it made no difference. Used cheap 20W50 oil and it still smoked. The problem is with the CI rings. I also build and sell race motors and have never had ring sealing issues until these CI rings.

Interesting. If anyone has a set they haven't fitted yet check free end gap and I guess someone else will be able to compare it to genuine Honda. They could just be made with too little tension?