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

[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.

[calj737]

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.

^^^Dead right.

[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.

[maxheadflow]

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.

Well a service limit of 0.25mm pretty much means you hone a .02 mm out and stuff in the next oversized piston..    Probably won't get it the cylinder round tho.  We used to recommend boring if the cylinder clearance was more then about 0.004" (about 0.1mm) at at that clearance the pistons will rattle a bit.   This is in Socal where temps can be on the warm side. 

 0.03mm almost 0.0015"   

The stuff I posted came right out of the service manual.

Here is a pic out of the MC service spec manual.   Maybe it's due to a fact that there is a typo? 

Other pistons (CB500/550) that are smaller state  new clearances as 0.01mm to 0.04mm, (0.0004-0.0016) as new bore fit.   I would think that 0.001 to 0.0012 is good for a CB350F (the spec book don't list piston clearance for the cb350F) It could be fit to 0.0004" but the tune would need to be spot on. 

[PeWe]

CB750 shop manual has also that small clearance mentioned where describing the procedures with pic 3-60.
 0.01mm to 0.04mm, (0.0004-0.0016)
That must be acceptable clearance piston to bore wall, not total, right? Total should in that case be double up as I see it.

[maxheadflow]

CB750 shop manual has also that small clearance mentioned where describing the procedures with pic 3-60.
 0.01mm to 0.04mm, (0.0004-0.0016)
That must be acceptable clearance piston to bore wall, not total, right? Total should in that case be double up as I see it.

I found the fit location in the manual here and in my own CB manual.   Thanks.   If you are thinking double because of 2 sides, that would be wrong, it's total clearance.  The low side is tight IMO.