Topic: Cb350f fresh top end smoking like nuts!

[bryanj]

A decent boring bar (van norman is one) will bore to 1 thou

[SquierA]

Quick update everyone. I installed the Honda NOS rings on the CI pistons, had the machine shop measure piston to to bore clearance and set a fresh hone. Got the bike back together, fired right up ran great and didn't smoke until it got to operating temperature and it started smoking again. I rode the bike for 10 miles varying throttle and engine loads and the smoke continued. It's not smoking as heavily as it did before but I think that's b/c we put 100 miles on the previous build and the head pipe and muffler had filled with oil that I cleaned out during the second tear down. Another odd item is the bike's valve cover/crankcase vent tube is pouring out smoke, almost more than what's coming out of the exhaust pipe. I will pull the plugs today to confirm all 4 are oil fouled then I think it's time for a leak down test to see if the issue is in the head.
The machine shop looked at the CI rings and they appeared to have a nice seal wear on them. We also installed another new set of valve stem seals just in case that was the issue. The bike has plenty of power (as it did on the previous build) so I'm thinking it's in the head. Also the bike runs great and doesn't smoke when it's cold. Once it heats up it smokes, won't idle and has heavy hesitation throttling up but has plenty of power. I think the poor idle and hesitation is from the the motor oil fouling the plugs at low rpms.
 
Just throwing out all that I can to this group to assist anyone with this same issue. This is crazy that we can't figure out the issue. I will keep posting updates as they happen.

[HondaMan]

A decent boring bar (van norman is one) will bore to 1 thou

Bryan:
Have you used the Van Norman bar yourself? I am toying with the idea of getting one.

[HondaMan]

Quick update everyone. I installed the Honda NOS rings on the CI pistons, had the machine shop measure piston to to bore clearance and set a fresh hone. Got the bike back together, fired right up ran great and didn't smoke until it got to operating temperature and it started smoking again. I rode the bike for 10 miles varying throttle and engine loads and the smoke continued. It's not smoking as heavily as it did before but I think that's b/c we put 100 miles on the previous build and the head pipe and muffler had filled with oil that I cleaned out during the second tear down. Another odd item is the bike's valve cover/crankcase vent tube is pouring out smoke, almost more than what's coming out of the exhaust pipe. I will pull the plugs today to confirm all 4 are oil fouled then I think it's time for a leak down test to see if the issue is in the head.
The machine shop looked at the CI rings and they appeared to have a nice seal wear on them. We also installed another new set of valve stem seals just in case that was the issue. The bike has plenty of power (as it did on the previous build) so I'm thinking it's in the head. Also the bike runs great and doesn't smoke when it's cold. Once it heats up it smokes, won't idle and has heavy hesitation throttling up but has plenty of power. I think the poor idle and hesitation is from the the motor oil fouling the plugs at low rpms.
 
Just throwing out all that I can to this group to assist anyone with this same issue. This is crazy that we can't figure out the issue. I will keep posting updates as they happen.

This one sure is odd(!). Do you know the valve stem-to-guide clearance in the head? It is very tight on this particular engine, due to the small size of the valve stems. During the middle 50% (or so) of valve travel, this engine's geometry will push the valve stem sideways by its max clearance, which often exceeds the speed (or flex) of the valve seals' material to keep up. I'm kinda wondering if this might be the culprit? It would rear its head much more when warm than when cold, and would cause high crankcase pressures and 'wet' crankcase exhaust fumes. Even one cracked valve guide would be enough to cause it. I have seen a CB400F head with a hairline crack in the head, radiating from a valve guide (exhaust), which made a real nasty mess of one exhaust pipe at the head, and it smoked out the crankcase hose.

[SquierA]

A guy posted a youtube video this summer with the exact same bike and issues, literally 2 top end rebuilds/teardowns with new pistons, rings and bore with the same results...smoking heavily. The valve guides were the culprit and he did a great job showing the problems in the video with a bore scope and head tear down.
I never had the valves, guides or head inspected and measured for wear. I just had the shop clean everything and install new valve stem seals. Stupid on my part. This bike had 23K miles on the clock when I bought it and the motor had been beaten up pretty bad.

I will take the head to a very reputable machine shop that specializes in vintage Honda engine restorations. I already spoke to the shop owner and he will inspect and measure the head, valves and guides. He's back logged about 2 weeks and I will post an update once he's completed his inspection.
 

[Tim2005]

Do you have a compression gauge? It would be useful to see what numbers you are getting- your comment "Another odd item is the bike's valve cover/crankcase vent tube is pouring out smoke, almost more than what's coming out of the exhaust pipe" makes me wonder, as the only time I have seen a small sohc4 engine (400f) breathing that badly it was due to low compression from broken piston rings. I've had very worn valve guides too in the past but they haven't really caused running problems.

[SquierA]

Quick update here. I took the head to a different machinist for a second opinion and watched him take it apart and inspect the valves and guides and everything was well within spec. I pulled the cylinders, pistons and rings and brought them to him as well and he is inspecting them for piston to cylinder clearance and all other specs to see if he can find the issue. As of now the solution is still not found. On a side bar we looked at the original pistons and rings and they don't indicate any issues with the severe oil pooling on the top of the pistons. I'm starting to lean towards the CI pistons being the culprit and am wondering if I need to order NOS Honda pistons at .75 and have the cylinders rebored to fit them. I will post another update soon.

[grcamna2]

Quick update here. I took the head to a different machinist for a second opinion and watched him take it apart and inspect the valves and guides and everything was well within spec. I pulled the cylinders, pistons and rings and brought them to him as well and he is inspecting them for piston to cylinder clearance and all other specs to see if he can find the issue. As of now the solution is still not found. On a side bar we looked at the original pistons and rings and they don't indicate any issues with the severe oil pooling on the top of the pistons. I'm starting to lean towards the CI pistons being the culprit and am wondering if I need to order NOS Honda pistons at .75 and have the cylinders rebored to fit them. I will post another update soon.

Do the valve guides check-ok ?

[SquierA]

The valve guides were well within spec. No issues there. No oil leaking down from the oil o ring under the guide either. The head and it's components checked out fine.   

[SquierA]

Another update. I shipped the cylinders, pistons and rings to Utah Crankshaft & ShortBlock and they measured the piston skirt to cylinder clearance and all 4 cylinders measured differently and were at the high end of being within the maximum wear limit. The machinist recommended 0.75 over NOS pistons and rings which I ordered today on ebay. Utah Crankshaft will rebore the cylinders to the NOS Honda pistons and we will see if that cures the smoking problem. I will post an update once I have the bike back together and running again.

[grcamna2]

Another update. I shipped the cylinders, pistons and rings to Utah Crankshaft & ShortBlock and they measured the piston skirt to cylinder clearance and all 4 cylinders measured differently and were at the high end of being within the maximum wear limit. The machinist recommended 0.75 over NOS pistons and rings which I ordered today on ebay. Utah Crankshaft will rebore the cylinders to the NOS Honda pistons and we will see if that cures the smoking problem. I will post an update once I have the bike back together and running again.

That will definitely cause it to smoke.
I'm glad you found NOS OEM oversize pistons and OEM piston rings.

[HondaMan]

Another update. I shipped the cylinders, pistons and rings to Utah Crankshaft & ShortBlock and they measured the piston skirt to cylinder clearance and all 4 cylinders measured differently and were at the high end of being within the maximum wear limit. The machinist recommended 0.75 over NOS pistons and rings which I ordered today on ebay. Utah Crankshaft will rebore the cylinders to the NOS Honda pistons and we will see if that cures the smoking problem. I will post an update once I have the bike back together and running again.

One of the largest problems I see with these engines after a 'reputable' machine shop worked on them is: most machine shops INSIST on using non-Honda measurements, substituting [some other measurement] instead.

Honda uses 0.0004"-0.0008" piston clearances in this (and CB400F/500F/550F) engines, This is usually 1/2 to 1/3 of what most (lazy, IMHO) machine shops consider to be "in spec", and your experience echos this, loudly. Most shops work in the 0.0020"-0.0025" arena for "clearances" without looking them up. Recently I helped another rider to fight his way through this problem with his machine shop when they insisted the piston bores were "...well within spec.." (this one in Canada) until the bike owner forced them to go and look it up themselves while he waited (he is also a cop...), which they did. When they came back, hat-in-hand, they then agreed to re-bore it to the CORRECT tiny clearances with new (next-step oversize) pistons, and it worked fine the next time...

[SquierA]

Another update here...
The previous machine shop had bored the cylinders to match the CI pistons and the piston to cylinder clearances were measured by Utah Crankshaft & Cylinder at #1 .00250, #2 .00375, #3 .00375, & #4 .00350.

I ordered the NOS .75 over Honda pistons and Utah Crankshaft re-bored the cylinders to match and now the clearances measure .00175 for all 4 cylinders. The factory spec is .0004-.0016 which means that technically the new bore is out of spec. I'm a little frustrated the shop didn't bore them within the spec since I provided it to them however they are very confident the previous bore was way out of spec and sloppy and feel this new set up will seal up well and the engine should not smoke. I will say the new bore and hone work are the nicest I've seen. There are no score marks, glazing or odd looking items in the cross hatch or cylinders. I'm very impressed with the visual look of the re-bore and hone.

I also obtained NOS .75 over Honda piston rings and will install this fresh top end over the holidays and will post another update when completed. 

Happy Holidays Everyone!

[grcamna2]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

[HondaMan]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

^^ Yeah. what he said...

I get a lot of top ends from these bikes, just to have my machine shop bore them correctly and install new guides (when needed) correctly. It's alot of postage, but those owners could not find the right shop, either.

Makes me sad.

[hawaiikid]

I can testify to Hondamans (Mark) clearance measurements. Local shop messed my bore up so I sent them to Mark and have no problems with smoke on my Cruzin Image pistons and rings on my 466 big bore cb400. I now have over 800 miles on new top end and no sign of smoke or any oil blowing out my exhaust. One happy camper here! I wonder if there should be a list of shops that can successfully do the bores on the smaller bikes as it seems that I've noticed a lot of smoking issues on the smaller fours.

[PeWe]

My CI pistons my CB750 K2 has are oversize +0.50mm pistons. Bore clearance on the loose side for CI.
I read about CI pistons on this forum and told the shop to bore max 0.025mm/ 0.000984251".

No smoking. I have another cylinder and new set of pistons. Tighter on them, think  0.02mm/0.000787401"

Forged pistons have the higher numbers, 0.05- 0.06mm.

A real waste that the shop could not do it right. Maybe tricky to find old OK sleeves to do another try on.

[HondaMan]

I can testify to Hondamans (Mark) clearance measurements. Local shop messed my bore up so I sent them to Mark and have no problems with smoke on my Cruzin Image pistons and rings on my 466 big bore cb400. I now have over 800 miles on new top end and no sign of smoke or any oil blowing out my exhaust. One happy camper here! I wonder if there should be a list of shops that can successfully do the bores on the smaller bikes as it seems that I've noticed a lot of smoking issues on the smaller fours.

I have been doing the 350F engines at 0.0004"-0.0006" clearance for a long time. Recently I did my own 750 to 0.0006" clearance on 3rd oversize pistons and the increased power and smoothness surprised me: it runs cooler, too. Top speed is higher as well. I also reduced the crank main bearing clearance to 0.0006"-0.0008", which is at/below Honda's spec, but I figured my crank was well-polished at 131k miles. That made the oil pressure almost 10 PSI higher on the hot engine, too: it used to run around 45-50 PSI at 2500+ RPM (and 5-7 PSI at hot idle, 900 RPM), now runs above 50 PSI at speed and around 10 PSI at hot idle. I took a long time to break it in, though, which was the tradeoff for going so tight. I'm doing another 750 like that now for one of our members.

[SquierA]

HondaMan I didn't know you offered this service or I would have reached out to you for the bore work I just had completed. In my restorations I most concerned about getting it done right and preferably the first time. Live and learn.

[HondaMan]

HondaMan I didn't know you offered this service or I would have reached out to you for the bore work I just had completed. In my restorations I most concerned about getting it done right and preferably the first time. Live and learn.

After I [painfully] "trained" my local machine shop to do it my way, they haven't messed any of these up. They have been good to work with, prices seem to be on-par with those I hear about here in the forums, around the US. The postage to-from is killer, though, especially since the Kung Flu thing started! I recommend Fedex (and insurance) for partial engines (others for whole ones) as the one who has damaged the least number of these shipments so far. UPS has virtually destroyed 2 engines and several top-ends that were sent, although one of the engines was a 550 in a simple cardboard box: when the UPS guy carried it in (!) and set it down, it rolled over, upside-down, because the box was essentially round by then. There were lots of broken fins and a dented-in clutch cover on that one.

[MRieck]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

^^ Yeah. what he said...

I get a lot of top ends from these bikes, just to have my machine shop bore them correctly and install new guides (when needed) correctly. It's alot of postage, but those owners could not find the right shop, either.

Makes me sad.

How are they not installing guides correctly? Not heating the head? Not bead blasting the carbon off? It's a pretty simple operation.

[HondaMan]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

^^ Yeah. what he said...

I get a lot of top ends from these bikes, just to have my machine shop bore them correctly and install new guides (when needed) correctly. It's alot of postage, but those owners could not find the right shop, either.

Makes me sad.

How are they not installing guides correctly? Not heating the head? Not bead blasting the carbon off? It's a pretty simple operation.

What seems to happen the most is: they severely over-clearance the valve stems after installing the guides (in my case, I install the gudes first, then have them do the work to match the guides to the seats with their Sioux and Serdi machines). I don't know where some of the shops get numbers like 0.003"-0.004" for 'correct' valve stem clearance, but it seems to be a popular number. These guides use half that figure, and tighter with bronze. I went tighter yet on my own engine in 2013, using Honda's old (1970s era) racing specs of 0.0010" intakes and 0.0016" exhausts in bronze guides, and it loves that.

Even my car engines use 0.0024" or less, so their work leaves me puzzled sometimes? How could they do it over an over and still be in business?

[MRieck]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

^^ Yeah. what he said...

I get a lot of top ends from these bikes, just to have my machine shop bore them correctly and install new guides (when needed) correctly. It's alot of postage, but those owners could not find the right shop, either.

Makes me sad.

How are they not installing guides correctly? Not heating the head? Not bead blasting the carbon off? It's a pretty simple operation.

What seems to happen the most is: they severely over-clearance the valve stems after installing the guides (in my case, I install the gudes first, then have them do the work to match the guides to the seats with their Sioux and Serdi machines). I don't know where some of the shops get numbers like 0.003"-0.004" for 'correct' valve stem clearance, but it seems to be a popular number. These guides use half that figure, and tighter with bronze. I went tighter yet on my own engine in 2013, using Honda's old (1970s era) racing specs of 0.0010" intakes and 0.0016" exhausts in bronze guides, and it loves that.

Even my car engines use 0.0024" or less, so their work leaves me puzzled sometimes? How could they do it over an over and still be in business?

I install first than have them honed using a Serdi pilot to check and achieve proper clearance. Yunick said .0025 for intake and .003 for exhaust for endurance racing automobiles and he admitted that that was loose. It is certainly loose for back then and way too loose for CB engine. I'm lucky as the shop I have used for many years works on motorcycle engines almost exclusively.
 .003 to .004 is worn out IMO.

[HondaMan]

Non-Powersports machine shops Piss Me Off:I have had Many situations like this.. 
I hope you have good experiences this time.

^^ Yeah. what he said...

I get a lot of top ends from these bikes, just to have my machine shop bore them correctly and install new guides (when needed) correctly. It's alot of postage, but those owners could not find the right shop, either.

Makes me sad.

How are they not installing guides correctly? Not heating the head? Not bead blasting the carbon off? It's a pretty simple operation.

What seems to happen the most is: they severely over-clearance the valve stems after installing the guides (in my case, I install the gudes first, then have them do the work to match the guides to the seats with their Sioux and Serdi machines). I don't know where some of the shops get numbers like 0.003"-0.004" for 'correct' valve stem clearance, but it seems to be a popular number. These guides use half that figure, and tighter with bronze. I went tighter yet on my own engine in 2013, using Honda's old (1970s era) racing specs of 0.0010" intakes and 0.0016" exhausts in bronze guides, and it loves that.

Even my car engines use 0.0024" or less, so their work leaves me puzzled sometimes? How could they do it over an over and still be in business?

I install first than have them honed using a Serdi pilot to check and achieve proper clearance. Yunick said .0025 for intake and .003 for exhaust for endurance racing automobiles and he admitted that that was loose. It is certainly loose for back then and way too loose for CB engine. I'm lucky as the shop I have used for many years works on motorcycle engines almost exclusively.
 .003 to .004 is worn out IMO.

Ah, yes, "Smokey" Yunick...I forgot all about him and his columns. I used to read his stuff until I got my first job in a Honda shop, under a master mechanic who knew these engines. He told me to "...forget everything you learned from Hot Rod magazines, the bikes are different..." on my first teardown under his guidance. Later on I learned he was dead right!

[SquierA]

Sorry for the long delay. I finished up the bike in June and stored it for the summer before starting it. I live in Phoenix Arizona and summer time is not when I'm spinning wrenches in the garage.

I fired up the bike this week and it doesn't smoke!!! Halleluiah!!! Turns out the piston to cylinder clearance created by the machine shop that bored the cylinders the first time was the culprit all along. This has been a long and very frustrating process (three top end tear downs searching for the culprit) and at the end of it I can confidently say that they key to preventing this from happening is finding the right machine shop that has experience boring these smaller cylinders. As I stated in my previous posts I had Utah Crankshaft and Shortblock rebore the cylinders using NOS Honda pistons at .75 oversize and I can't thank them enough for getting it right. The turn time was very quick and the price was reasonable.   

Thanks to everyone for your input and assistance. I wanted to circle back and post this solution to assist other restoration mechanics from going through the frustration of chasing down a problem that could have been prevented by using a machine shop that has experience with these smaller inline fours. 

Now it's time to log some miles on this little CB350F and seat the rings!