Author Topic: Forged Pistons for a CB350F (Read 6269 times)

camshaft1991 · « **on:** November 10, 2015, 08:22:01 PM »

I've been rebuilding a CB350F engine, from the crankcase up, for over a year now and until a couple of months ago I had to source my build out to someone else due to school and work. The mechanic who was originally fixing my wiring harness found out that it was leaking oil from cylinder number one below the cylinder head. The oil streams out more as the bike warms up. It can not idle at 1200rpm. In my efforts of rebuilding my bike I used a flex hone drill tool to hone my cylinders. It has a series of course beads that are used to hone and clean cylinders, however, I was informed that I used the wrong tool for the job. My pistons are original with exception of new rings that I installed, but now my compression suffers across all the cylinders equally. They read at 90 psi. The engine has not been torn down yet for further inspection, but my mechanic and I both agree that another honing might night fix things. The more likely option is to go with an overbore.

It really sucks how all my efforts of accumulating parts and piecing them together for over year almost seems like a waste. After we find out what overbore we need to go with I was thinking about buying a set of pistons and rings through CMSNL. My mechanic suggested that I upgrade to forged pistons. He referenced me to the Wiseco, a company that specializes in custom piston manufacturing.

http://www.wiseco.com/

Wiseco can manufacture forged pistons as small as 39mm in bore but they have a minimum of 12 per order. They have a sister company, JE Pistons, that makes custom small orders, but the smallest piston they can manufacture is 58mm.

For those who do not know, the STD pistons on a CB350F are 47x50mm. I'm not trying to build a track bike, but I want to add a little longevity to it. I've done some research on cast vs forged pistons and found out in most aspects (all) that forged is better than cast. When I receive my quote, to see if it is worth the investment, I want to possibly include others in placing an order. Any helpful input on my bike or on forged pistons would be helpful

Powderman · « **Reply #1 on:** November 10, 2015, 08:38:18 PM »

If it's just going to be a street machine forged pistons are an unnecessary expense. If you just need an over bore I would just get the CruizinImage kit off Ebay for $120 and be done. You end up with a 393cc motor. CMSNL will run you close to $400 for a smaller motor.

camshaft1991 · « **Reply #2 on:** November 10, 2015, 09:08:38 PM »

How big are the individual pistons? I found some on ebay, but I can't see what bore the individual pistons are. I know there is a gain with power when increasing the bore, but is there any bad tradeoffs when the bore is increased dramatically? (Aside from tuning)
Will the bike be more susceptible to overheating?
Will I be more likely to blow a headgasket in the future?
Will the larger pistons put more stress on my connecting rods and crankshaft?

Powderman · « **Reply #3 on:** November 10, 2015, 09:14:27 PM »

The CI kit is 50mm pistons, a CB400f is 51x50. The CI kit makes the motor "square" which is a good thing. I've never heard anything bad about the CI kits. I wouldn't have any concerns using this kit.
http://www.ebay.com/itm/72-74-Honda-CB350F-393cc-Bigbore-Pistons-Kit-50mm-W-Head-Gasket-CI-CB350FBB-/141762219654?hash=item2101afce86:g:wwMAAOSw9NxTvmB8&vxp=mtr

Flyin900 · « **Reply #4 on:** November 10, 2015, 09:15:23 PM »

These bikes are really suitable mostly for lower/medium speed riding IMO. I restored one earlier this year and found it pretty buzzy in the 50-60 mph range. Fun to ride, yet I had enough of it after a month and sold it. It could have used another gear, yet it lacked the power for an additional gear I am sure.

So it depends on what you want to spend and how you will be riding it. I agree with the previous poster, spend your money on the Cruzin Image kit. They have a good reputation and value for the money.

Garage_guy_chris · « **Reply #5 on:** November 10, 2015, 09:16:07 PM »

Out of curiousity i just had a look and crusin image has both 0.50mm over

http://www.ebay.com/itm/72-74-HONDA-CB350F-0-5mm-47-5mm-OVERSIZE-PISTON-SET-Four-Pistons-are-included-/141192625953?hash=item20dfbc7f21:m:mcnX1IALGqLxheZTkntD_QA&vxp=mtr

** these would be good if you don't want to deal with the big bore stuff

and 393cc 3.00mm over

http://www.ebay.com/itm/72-74-Honda-CB350F-393cc-Bigbore-Pistons-Kit-50mm-W-Head-Gasket-CI-CB350FBB-/141762219654?hash=item2101afce86:g:wwMAAOSw9NxTvmB8&vxp=mtr

Powderman · « **Reply #6 on:** November 10, 2015, 09:20:00 PM »

Quote from: Garage_guy_chris on November 10, 2015, 09:16:07 PM

Out of curiousity i just had a look and crusin image has both 0.50mm over

http://www.ebay.com/itm/72-74-HONDA-CB350F-0-5mm-47-5mm-OVERSIZE-PISTON-SET-Four-Pistons-are-included-/141192625953?hash=item20dfbc7f21:m:mcnX1IALGqLxheZTkntD_QA&vxp=mtr

** these would be good if you don't want to deal with the big bore stuff

and 393cc 3.00mm over

http://www.ebay.com/itm/72-74-Honda-CB350F-393cc-Bigbore-Pistons-Kit-50mm-W-Head-Gasket-CI-CB350FBB-/141762219654?hash=item2101afce86:g:wwMAAOSw9NxTvmB8&vxp=mtr

These are big bore stuff, nothing more to deal with than any other kit. You still need to bore the jugs. For $13 more why not go with the larger displacement?

camshaft1991 · « **Reply #7 on:** November 10, 2015, 09:43:05 PM »

Most of my commuting would take place in the city, New Orleans. Aside from the interstate and a couple of express ways the speed limits here does not exceed 35mph. We also have lots of potholes so going any faster can be dangerous for any motorcyclist. I might consider cruising around my state and if 70mph is the cruising I am fine by that . If I was presented a CB400f instead of a CB350F I would of bought it but one thing in particular about this bike is that it can fit on a motorcycle carrier that mounts on a truck hitch. I've piggy backed it before with a 3/4 truck and my motorcycle carrier is rated for 500lbs while the bike is around 400lbs wet.

I appreciate all the input fellas. This all seems a lot more easy to deal with financially than with CMS or forged pistons. I've seen these piston sets before on ebay but I did not know if they were any good.

Powderman · « **Reply #8 on:** November 10, 2015, 09:59:26 PM »

Out of 6500 feedbacks this year only 14 were negative and none of those were about the CB big bore kits. They are reputable and you should be happy with the results.

Garage_guy_chris · « **Reply #9 on:** November 10, 2015, 10:41:39 PM »

The 750 big bore kits are fine, i have one in my 750 and its working great. I have been having some trouble with sitting in traffic and overheating but its a 750 and it also has other mods, so were talking apples and oranges. But for reference the route i regularly take that overheats the 750 also mildly overheats my bone stock CB450 so.... Anyways its food for thought

here is the big thread about the 65mm 836cc kit for the 750's. it might provide some more insight or opinions for you
http://forums.sohc4.net/index.php/topic,89779.0.html

the one difference with the big bore to the 0.50mm over kit is the cylinder boring is going to cost more as they have to make more passes = more time. Also your gonna have to consider things like Conrod strength, quench bands, piston to head clearance, also big bores are like a gateway drug to things like head porting, cams, exhausts, bigger carbs and so on. I wish i knew the CB350F's so i could better advise.

Powderman · « **Reply #10 on:** November 10, 2015, 11:18:25 PM »

Around here boring a hole is boring a hole, same price (approx $50 a hole), no matter how many passes. I figured boring a 2" hole is a lot easier than a 4", no difference in price.

Hurstman1972 · « **Reply #11 on:** November 12, 2015, 04:13:15 PM »

I'm rebuilding the top end on my '74 350F, and will need a set of Pistons in the 2nd overbore size (47.5mm). If that's the size you're going for, I might consider cost sharing. Post the Wiseco quote and I'll let you know.
Thx!

camshaft1991 · « **Reply #12 on:** November 23, 2015, 01:11:25 AM »

If anyone wants to considering Weisco , which I'm not anymore, it is $110-$117 per piston for an order of 12 or more depending on optional coatings.

Don R · « **Reply #13 on:** November 23, 2015, 10:31:46 AM »

Back to the beginning, does it burn oil or run poorly? A tiny engine on a compression tester with a long hose can give an unusually low reading.

754 · « **Reply #14 on:** November 23, 2015, 01:27:40 PM »

Powderman. Nobody around here will take out 3 or 4 mm in one cut.. So extra cuts...
I think you will find it so almost everywhere..
3 mm cut..the sleeve would probably start spinning..

Sportsters to go to 1200 from 883, you take out a half inch.... 3 cuts on a lathe.... Take forever on a boring bar..

Powderman · « **Reply #15 on:** November 23, 2015, 04:57:32 PM »

Quote from: 754 on November 23, 2015, 01:27:40 PM

Powderman. Nobody around here will take out 3 or 4 mm in one cut.. So extra cuts...
I think you will find it so almost everywhere..
3 mm cut..the sleeve would probably start spinning..

Sportsters to go to 1200 from 883, you take out a half inch.... 3 cuts on a lathe.... Take forever on a boring bar..

Nobody around here charges by the cut. You can only go so far making cuts. The place I use charges per hole, not how many passes it takes.

754 · « **Reply #16 on:** November 23, 2015, 06:05:41 PM »

Have you done bigbores.. If not ask them how much to take out 4mm like for an 836..
Automotive boring bars are designed for light cuts for reconditioning, not hogging major amounts of metal..like z1/2 inch on the Sporty jugs..
I know a few guys with engine shops, they like to charge by each pass.. They had me bore some Sporty jugs on the lathe cause it was too time consuming on their equipment..

Powderman · « **Reply #17 on:** November 23, 2015, 06:23:02 PM »

Checked prices for boring my CB400f cylinders for 466 kit. I figured it would be cheaper because the holes aren't much more than 2". That's when I was told $50 a hole no matter the size.

754 · « **Reply #18 on:** November 23, 2015, 06:25:43 PM »

Ah i see no .. We used to get 25 or 30 a hole..
Be a whole lot of cussing and looking for new cylinder , if the sleeves spin while boring..

jimbojangles · « **Reply #19 on:** November 24, 2015, 01:59:53 PM »

I'm in a similar place with a 350f. Tried all summer to avoid a tear down, but can't get it to stop burning oil and need to replace the head gasket anyway.

I've compared the big bore and standard oversize kits and can't decide. Would the big bore pretty much require me to get an oil cooler? I live in SEC country. I'm also wondering how much I'll regret going with the standard overbore every time I twist the throttle. I guess it's a debate of instant pleasure/fun vs. longevity/peace of mind.

I'm guessing the big bore will give nearly the same power as a stock cb400f? I wonder what I could get out of it with a cam? I think I like the idea of my little 350f surprising people. I've owned much faster bikes, but I like riding this thing more than all of them.

Powderman · « **Reply #20 on:** November 24, 2015, 02:07:12 PM »

I've not heard of any overheating problems with the CruzinImage 466 kit. More power will be had from porting the head than from just a cam change if you don't let the motor breathe better.

camshaft1991 · « **Reply #21 on:** November 27, 2015, 02:14:00 PM »

Quote from: Don R on November 23, 2015, 10:31:46 AM

Back to the beginning, does it burn oil or run poorly? A tiny engine on a compression tester with a long hose can give an unusually low reading.

I've only been told over the phone of the symptoms but I want to say that it burns oil (smoky exhaust) but it definitely leaks oil and has low compression of 90 psi over all the cylinders. I tried honing my cylinders myself and the drill press I used had a an ever so slight wobble and it turned my cylinders to ovals.. I've been informed that .25 bore over or more would be sufficient to correct my botched honing job. Also even if the pressure gauge was inaccurate the bike cannot drop below 2500 RPM without stalling out. It is supposed to idle at 1200 rpm

harisuluv · « **Reply #22 on:** November 27, 2015, 02:42:09 PM »

Most people that use a flex hone just use it in a hand drill. If you think that a slight wobble in a drill press ruined your cylinders, I think you're being fed a load of BS.

Also now your diagnostics and symptoms are now being done without you by someone who will directly financially gain by more things being wrong with your bike. From what I've read, I don't trust your mechanic at all.

"It burns oil (smoky exhaust)" - What color is the smoke? How does he know it's burning oil?

"it definitely leaks oil" - From where? Head gasket? Where is this leak exactly?

"has low compression of 90 psi over all the cylinders" - So when you "ruined" the cylinders, you ruined them all exactly equally? sounds like the bike wasn't warmed up or the test wasn't done with throttle wide open. And what were the results of the test after oil was added??

"the bike cannot drop below 2500 RPM without stalling out. It is supposed to idle at 1200 rpm" - And this is correlated to your cylinders because... why?

Very very sloppy troubleshooting by your "mechanic." If there is logical reasoning going on here, I don't see it, and don't follow.

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Author Topic: Forged Pistons for a CB350F (Read 6269 times)