Topic: CB350F Big Bore Rings?

[eelrider]

Hey everybody, my cb350f started smoking during a track day (got black flagged). Local Honda shop did a leakdown test and said valves seemed good but compression was a bit low. I bought and installed Cruzin Image second over piston kit. Smoke got worse. Decided to go Cruzin Image “big bore” 393cc. Had barrels bored by reputable shop in Canada and assembled. Smoked like a James Bond car. Oil on all 4 piston crowns. Blamed headgasket.

Installed copper headgasket still smoking like mad. Tried an oem head gasket still smoked like mad. Had barrels measured at a local shop who is experienced with the tight tolerance of little bores, and all looked good. Installed a second CI big bore set hoping it was a bad set of rings. Oil burning is worst yet, even fouling plugs, making for a ridiculous 5 mile test ride.

My testing and reading leads me to believe that CI needs a better source of rings. I’ve got a parts bike available with 30k. Probably trying to go back to 347cc with oem parts. But before I buy any more stuff $$$$. Has anyone found good quality rings, that are an accurate fit, to the 50mm CI pistons? Thanks, in advance.

[Don R]

 Some racing ring companies can sell by size, I'd look into that.

[Don R]

 Huskvarna and stihl chainsaws use 50mm pistons, some Yamaha and I'm sure others, the chainsaws only have two rings. I looked on ebay

[HondaMan]

What piston clearance did your machine shop install? the 350F should have 0.0004"-0.0006" (that is 4 to 6 ten thousandths of an inch). If they installed typical car-engine clearances like 0.002" (2 thousandths), that is the same as a severely worn-out engine in this size.

[ekpent]

 Been other guys on the forum who have had terrible luck boring out the little 350's if you can find them in the search. Their bikes turned into smokers also. Think one guy gave up and sold his. I put scraper rings in upside down on a 750 once and made quite a smoker 

[markreimer]

This has got to be at least the 6th or 7th cb350f cruzin image 393 smokefest failed engine. I have yet to even hear of a successful use of these pistons save one, where the owner uses different rings.

I had the exact same experience. Had my bores measured by a second shop, and confirmed the clearances were well in spec.

The rings are garbage. You may need to source some from a ring company. Get them to make you a few sets and you could sell them to members easily I’d expect, make some of your money back.


Sent from my iPhone using Tapatalk

[Mark K]

A few months back, I was on the hunt for replacement rings for big bore pistons (836? 899?) a PO had installed in the project '73 CB750K3 I bought. Somebody here suggested Total Seal Piston Rings. I emailed them (quick response) and later spoke with Lake Speed on the phone, he was very knowledgeable. Although they weren't able to help me, they might be able to help you.

Total Seal Piston Rings
https://www.totalseal.com/

TS Contact page
https://www.totalseal.com/contact

[eelrider]

Thanks for the ideas. I was hoping to find someone who used rings from a source (x) and part number (x) and they actually worked on the 50mm pistons. Without a confirmed success I think it best to give up on the 393cc.
To answer Hondaman’s question, I don’t know the actual piston to cilinder clearance. The Canadian shop who did the bore does rebore work for my local Honda shop. The smaller local machine shop who checked bores and pistons told me the clearance was correct (unfortunately no numbers) bores were round and pistons looked good but rings looked strangely worn, having only 50 miles.
It is of course possible the bore was too large (note; ring gap was spot on)
 When I disassemble next, maybe I can get accurate measurements on the 50mm bore and and determine if clearance was too large and maybe CI isn’t the problem.
Again, thanks for chiming in.

[markreimer]

Thanks for the ideas. I was hoping to find someone who used rings from a source (x) and part number (x) and they actually worked on the 50mm pistons. Without a confirmed success I think it best to give up on the 393cc.
To answer Hondaman’s question, I don’t know the actual piston to cilinder clearance. The Canadian shop who did the bore does rebore work for my local Honda shop. The smaller local machine shop who checked bores and pistons told me the clearance was correct (unfortunately no numbers) bores were round and pistons looked good but rings looked strangely worn, having only 50 miles.
It is of course possible the bore was too large (note; ring gap was spot on)
 When I disassemble next, maybe I can get accurate measurements on the 50mm bore and and determine if clearance was too large and maybe CI isn’t the problem.
Again, thanks for chiming in.

You might be able to use 400F pistons, but would be on the ragged edge of cylinder thickness. Would need an oil cooler I’d expect.

Mine were done to 0.0004” and checked at two machine shops. Still smokey like a chimney. Tons of oil pooled on the piston tops.


Sent from my iPhone using Tapatalk

[scottly]

Honda CT/Atc 90s had a stock 50mm bore, and there may be other bikes that use 50mm bores, either stock or as overbore sizes. If the rings are the same thickness, they may work with the CI pistons. The problem seems to be with the CI rings, rather than the pistons.

[eelrider]

Finally had time to assemble. Lightly ball honed barrels from a donor bike, then installed original pistons with NOS Honda rings in stock size (from CMSNL). Also used Honda head gasket and base gasket for good measure. Bike started great with no smoke. Been on a couple 20 mile rides with no smoke.

[HondaMan]

Finally had time to assemble. Lightly ball honed barrels from a donor bike, then installed original pistons with NOS Honda rings in stock size (from CMSNL). Also used Honda head gasket and base gasket for good measure. Bike started great with no smoke. Been on a couple 20 mile rides with no smoke.

Do you still have the old cylinders and the CI pistons? If so, would you like to attempt a measurement? It would go like this:
1. Invert the cylinders (deck side down).
2. Remove the rings from a piston.
3. Lay a 0.0010" feeler gage next to the front of the piston's skirt (at the bottom of the skirt) and try to push it into the bore.

This is a good way to find out what the piston clearance actually is. The front or back of the piston skirt is the widest spot on the piston, so it should NOT let you push the (piston + feeler gage) into the bore.

If you CAN push them together into the bore, they bored it too big, with too much clearance. Anything more than 0.0016" clearance in this little engine will smoke big time: that is way too much. The OEM spec is 1/4 (25%) of that much.