CB350 - to bore or not to bore. that is the question.

[iContango]

So because my kickstart gear popped off, requiring me to split the cases, I figured why not disassemble the top end...of a perfectly fine running engine. I probably shouldnt have, but I already have the damn thing apart and am looking for opinions on to overbore, or not.

I am thinking about just sending the jugs to Boretech for a 65.5mm bore & hone job, plus picking up their pistons & rings. This will cost north of $300 with a couple weeks of down time.

My second option is to keep things as-is and just replace the rings. That will cost me much less of course, with little down time.

What do you think of my pistons and cylinder walls? Supposedly the cylinders have been honed recently, although I cant confirm that. Cross-hatching is still present, but on the right cylinder I see what appears to be embedded water stains - not sure what that is about. Any advice would be great!

 

[Sonreir]

What is your goal with the build?

1.5mm over on the bore isn't a whole lot. It'll only bump you up 15cc, or about 4%.

 

[iContango]

What is your goal with the build?

1.5mm over on the bore isn't a whole lot. It'll only bump you up 15cc, or about 4%.

My goal is mainly reliability and have a nice running bike. I pretty much have that right now with my current setup (stock engine & carbs w/ stock jets, pamco ignition, mikes xs coils, ballistic battery, Kohler reg/rec, shorty muffs, uni filters), but need to swap out the engine since it is leaking oil everywhere. I had this engine sitting around from another bike and I figure why not just rebuild it, then do the swap in one day. I also have Mikuni VM30s laying around that I need an excuse to set up.

Any thoughts on how much more power I would get with the same setup, only variables changing would be the overbore + Mikunis?

 

[surffly]

Have you used a micrometer and a manual to see if your bores and pistons are within spec?
Why go with the HC 1.5mm over bore techs if your goal is just to ride?
Bet you can get a .25 or .5mm OEM or OEM replacement set up for less money.

 

[Jramos13]

if you decide to bore them i have a pair of 65.5 pistons from bore tech with less than 500 miles, you just would need a new set of rings, bore tech sells them at $27 each set i believe, i also have the oversize head gasket, not sure if they are reusable but it does looks good, if your interested i can do $60 for all shipped

 

[Sonreir]

My goal is mainly reliability and have a nice running bike. I pretty much have that right now with my current setup (stock engine & carbs w/ stock jets, pamco ignition, mikes xs coils, ballistic battery, Kohler reg/rec, shorty muffs, uni filters), but need to swap out the engine since it is leaking oil everywhere. I had this engine sitting around from another bike and I figure why not just rebuild it, then do the swap in one day. I also have Mikuni VM30s laying around that I need an excuse to set up.

Any thoughts on how much more power I would get with the same setup, only variables changing would be the overbore + Mikunis?

VM30s may add a pony or two at the top end, but you're also going to lose a little bit of throttle response (and maybe power) at the lower RPMs. VM30s are a bit big for the CB350. VM28s are a better match.

4% increase in displacement is about a 4% increase in torque. That works out to about two horsepower at redline.

So displacement increase and carbs will get you around two or three horsepower.

 

[iContango]

Have you used a micrometer and a manual to see if your bores and pistons are within spec?
Why go with the HC 1.5mm over bore techs if your goal is just to ride?
Bet you can get a .25 or .5mm OEM or OEM replacement set up for less money.

I have not, I only have a digital caliper, not sure if that is precise enough.

I'd go with the Boretech setup because if I am boring the pistons, why not make them as big as possible without having to do other upgades. I am also put at ease knowing everything is in the hands of boretech as opposed to some random shop...

 

[iContango]

VM30s may add a pony or two at the top end, but you're also going to lose a little bit of throttle response (and maybe power) at the lower RPMs. VM30s are a bit big for the CB350. VM28s are a better match.

4% increase in displacement is about a 4% increase in torque. That works out to about two horsepower at redline.

So displacement increase and carbs will get you around two or three horsepower.

So pretty negligible then... Hmm maybe I should rethink this. Any thoughts on the "water stains" present on my right cylinder wall? Think I need a hone? What about the black soot on top of my pistons? If everything measures to spec I may just put in some new rings and reassemble.

 

[VonYinzer]

Clean, hone, reassemble. Unless the bores are screwed up or you're planning on a real deal go fast motor, anything else is a waste of money.

 

[Sonreir]

Clean, hone, reassemble. Unless the bores are screwed up or you're planning on a real deal go fast motor, anything else is a waste of money.

+1

But I would add that getting a spec on the cylinders is a good idea before you reassemble everything. The previous hone job looks like it may have been done by hand, so getting measurements before you reassemble would be a good idea. Don't worry about the stains.

 

[iContango]

Great feedback guys, much appreciated - I will likely just get everything checked out and then reassemble.

Think I can get by without a hone job and/or ring replacement, or is it always a good idea to do both whenever it is disassembled? The rings that are on there are nearly new (I was the dummy who took the engine apart even knowing this).

 

[Sonreir]

If you have the time, money, and inclination, it won't hurt.

You wouldn't be the first guy to throw new rings on and call it good though.

 

[Kanticoy]

Measure the end gap on your rings when installed to determine wear. The limits are listed in the manual. I would get a bore measurement just for peace of mind. A good ball hone after that and call that part good. Since it is apart, de-coke the combustion chamber on the heads, lap the valves, clean the pistons, add new valve guide seals (vitton for longevity) then reassemble. Your only expense will be the valve guide seals and a gasket kit (valve guide seals usually come in a top end kit) if everything measures to spec. You'll know at that point that your motor is in top notch shape and you can ride confidently.

 

[iContango]

Measure the end gap on your rings when installed to determine wear. The limits are listed in the manual. I would get a bore measurement just for peace of mind. A good ball hone after that and call that part good. Since it is apart, de-coke the combustion chamber on the heads, lap the valves, clean the pistons, add new valve guide seals (vitton for longevity) then reassemble. Your only expense will be the valve guide seals and a gasket kit (valve guide seals usually come in a top end kit) if everything measures to spec. You'll know at that point that your motor is in top notch shape and you can ride confidently.

Do you have any suggestions on the best method for de-coking the combustion chamber on the heads? Any solvents that work well, type of brush, etc? The same goes for cleaning the pistons - any tips on what to do (and not to do)?

I have a full gasket set with valve guide seals all ready to go, so really just need to get this all figured out then Ill be ready for assembly.

 

[Sonreir]

Zep Orange is good, but don't leave it on too long. It can be a bit aggressive with aluminum.

Start with soap and hot water and a plastic scrub brush. If it still isn't clean, then try something a bit stronger.

 

[VonYinzer]

A light hand and a brass scrub brush will work great. Any good degreaser will work, but as Matt said... Be careful. Some will ruin alloys. Also...

DO NOT USE OVEN CLEANER, NO MATTER WHAT ANYONE SAYS!!!!

 

[Rgrpfdr1]

Re: Re: CB350 - to bore or not to bore. that is the question.

DO NOT USE OVEN CLEANER, NO MATTER WHAT ANYONE SAYS!!!!

What is the effect of using oven cleaner? Just curious, not trolling. I used it to remove the blue anodizing on a set of aluminum rims once.

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[mydlyfkryzis]

Oven Cleaner is Sodium Hydroxide, also called LYE. It dissolves aluminum....

If not removed and anything left neutralized, it will corrode aluminum...

Strong Acids and Strong Alkali (Lye is an alkali) really corrode aluminum quickly....

 

[Rgrpfdr1]

Re: Re: CB350 - to bore or not to bore. that is the question.

Oven Cleaner is Sodium Hydroxide, also called LYE. It dissolves aluminum....

If not removed and anything left neutralized, it will corrode aluminum...

Strong Acids and Strong Alkali (Lye is an alkali) really corrode aluminum quickly....

Thanks!
I washed and sanded mine down afterwards. They've since been painted. I am having them chemically stripped and powder coated this weekend though.

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