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

[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?

[markreimer]

I think I must have read at least a dozen threads on SOHC4 with badly smoking cruzin image 393 kits by now. In fact I don’t think I’ve read a single successful build with this piston kit yet.

Moral or the story - don’t buy them. They’re crap.


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[Smithy]

I found a guy who had the CI 393cc kit fitted and his bike doesn't smoke at all. I had my cylinder bores rechecked on my bike and they were slightly oval and not true from boring. Apparently such small bores are difficult to machine to Honda specs. I will be avoiding general engine shops from now on and looking for a motorcycle specific machine shop.

[disco]

Hi Mark,

I just had smoke issues with a 750 rebuild. Same symptoms as yours ie chucking oil down the exhaust. My engineering shop measured it & said I had 3 bent rods (ever so slightly). It was a 'Frankenstein' motor, so It was a collection of parts that I didn't know the history of. Anyway I'll know for sure if cured, in a couple weeks time, when I get a chance to re-assemble it.

I'm wondering whether bent rod/s was the problem with your 350/4? There's no way I couldn't tell anything wrong, by looking at the rods. They looked fine to me.

[dave500]

man ive assembled a few bitza 550 motors using new valve stem seals only and none have done nothing but run well,ive swapped pistons and used rings as sets into barrels etc and heads around,i make a point of checking ring gaps and some have been a little "wrong" but still good running motors I expected to smoke a little,my spare motor is technically a piece of #$%* but it flys like my rebored one?both have new chain pairs in them.

[disco]

Hi Dave,

I've done the same thing many times, and maybe I lucked out mostly?

 I've just had same symptoms as Mark R with his 350/4 and it occurred to me that possibly his pistons weren't at fault. He did say he had same issues with OEM & CI Pistons. That suggests to me that the culprit is not pistons/rings but something else like a bent rod?

[crazypj]

Because the 350 bore is small, you would be better off finding someone who can just hone it to size without boring. Using 80 grit hone will get close to size pretty quick. You need a rigid hone though (AMMCO, SUNNEN, etc) not he 3 leg glaze buster
With a rigid hone getting to wiihin 0.002mm is dead easy,even with a drill motor version (I used to teach it)
I have a set of CB400F liners ready to go into the 350 I have in shed, not sure how much I'll need to bore bven with a drill motor version (I used to teach it)
I have a set of CB400F liners ready to go into the 350 I have in shed

[dave500]

honing from stock to even say 1st oversize @.25 isn't what you do full stop!the bores wont be square or parallel,let alone straight and true even with a rigid hone,you need a machinist who gives a #$%* and applies attention to detail.

[Redline it]

this was a sequel in a mini series. has there been another follow up on it yet? i got to page 8 and skipped to 10 finally. whatever you got to get this much input is something to hang your hat on. after my second or 3rd question on a matter like this, the lines go dead. of course it could be the replies hit the nail on the head. like right to the solution. hope i didn't miss the part if you ever seated the rings or found the smoky problem.  i did see your review on the cruize kits are no bueno. i got one setting in a spot ready to put in over the next few years, a 466 kit. i'll make sure the barrels are pretty much on their last leg before cutting that much out.

[Kevin]

I was just reading an article on Mike Nixon's website that mentions the Cruizin Image piston kits.
I'll sum it up to say that his opinion is the rings are garbage but the pistons are good.
Here is a link to the article, it is worth the time to read it.
https://www.motorcycleproject.com/text/cylinders_done_right.html

[HondaMan]

Interesting.
I've never had any problems with the CI pistons, not in any of these engines from the Baby four thru the 750, in all sizes from step-bore to big-bore. But, my machine shop holds my required tolerances: the 350F uses 0.0004" piston clearance and the 750 [normally] uses 0.0008" in my builds (except my own: it is built to 0.0006"). The process is as follows: bore to required size minus 0.005", hone to finished size. Too many machine shops don't do it this way...

[dave500]

its a lost art,decent old school knowledgeable ones hardly exist now?they aint carefull and lack expertise and attention to detail,not that I can do it but they are supposed to be the experts,theyll blame the failure on your assembly/break in?#$%* ive been thrashing the #$%* out of a david silvers 1st oversize 550 kit for more than a few years now no trouble at all,and I mean thrashing!a mate has a shop does all that stuff,he did mine,each piston came back lettered to a bore(save any wrongness in cylinder order compared to numbers),i didn't spare the break in either just more or less went for it with a coupla real short oil/filter changes.

[PeWe]

Interesting.
I've never had any problems with the CI pistons, not in any of these engines from the Baby four thru the 750, in all sizes from step-bore to big-bore. But, my machine shop holds my required tolerances: the 350F uses 0.0004" piston clearance and the 750 [normally] uses 0.0008" in my builds (except my own: it is built to 0.0006"). The process is as follows: bore to required size minus 0.005", hone to finished size. Too many machine shops don't do it this way...

That's not  tight?
0.001- 0.0012 (0.025-0.030mm) is too big clearance for the CI cast pistons or cast pistons in general? Total clearance bore minus piston OD at skirt.

I have a cylinder that will be bored for CB750 CI +0.5 mm pistons. 392 version for a little bit more compression. 120USD at www.cruzinimage.net where all parts are available, eBay not.

[dave500]

check the pistons first,measure them each in three axis,theyll vary,the stock Honda ones would vary very little,quality!

[maxheadflow]

Interesting.
I've never had any problems with the CI pistons, not in any of these engines from the Baby four thru the 750, in all sizes from step-bore to big-bore. But, my machine shop holds my required tolerances: the 350F uses 0.0004" piston clearance and the 750 [normally] uses 0.0008" in my builds (except my own: it is built to 0.0006"). The process is as follows: bore to required size minus 0.005", hone to finished size. Too many machine shops don't do it this way...

That's not  tight?
0.001- 0.0012 (0.025-0.030mm) is too big clearance for the CI cast pistons or cast pistons in general? Total clearance bore minus piston OD at skirt.

I have a cylinder that will be bored for CB750 CI +0.5 mm pistons. 392 version for a little bit more compression. 120USD at www.cruzinimage.net where all parts are available, eBay not.

Boy, you guys like to set them up tight. I used to set them at 0.0015.. Specs seem to indicate that.  The 79 ups, you could run tighter as I suspect they were hypereutectic.   

[PeWe]

That spec for CB750 say 0.025-0.055mm
0 0015" =  0.0381mm
Service limit 0.25mm!

My 836 cylinder ran @ 0.2mm  clearance without smoking.
(83whp)
That is rebored to 849cc, clearance agreed around 0.03mm. I have not measured to verify.