Overpressurization of CB350 Honda crankcases

P

pacomotorstuff

Coast to Coast
Hi everyone,
I know a lot of you browse the VRRA site (www.vrra.ca) and there's a lot of good info applicable to the bikes we build.
The CB350 vintage race bikes seem to experience overpressurized crankcases at racing rpms, causing among other things, oil seals to blow, then leaking, crashing, etc. One of the things required this year is mechanical retention of the clutch pushrod seal on the left side of the engine as that has been the main culprit.
The retainer seems like a straightforward enough modification and I'm going to do it to my 350 and also my 175 twin. There's a good discussion on various ways of venting the crankcases and again, I'll have extra venting for both of my bikes.
So, I wondered whether or not any of you have experienced simiar problems with your rides, Honda or not?
For a full read and to look at some photos of the retention device, go to the VRRA website, General Discussion, the topic is "All Honda CB350 riders".
Regards,
Pat Cowan
 
Not an uncommon spot for a leak, but that sounds suspiciously like trying to cure the symptom rather than the cause. If the pressures are getting high enough to blow seals, then that implies a good deal of pumping losses as well. I'd focus on venting the cases first, then worry about the seal later. Possible ring seating issues as well?
 
Yea I totally had this problem on so many occasions. But I hear its more an issue of blow back into the crank case from worn rings than bad seals. Last time this happened I was ridding WOT over the Vincent Thomas bridge in San Pedro and didn't notice until I was in the other side of long beach. Hardly had a drop of oil left. Then for a different reason, the engine seized up. Little to say, it's been 5 months since that day and I'm still undergoing a rebuild which started with fixing that darn seal!!

freedomgli here on the site made a pretty clean brace for his cb350. here is a link to his build

http://www.dotheton.com/forum/index.php?topic=9574.50

I hope he doesn't mind if I link his photo up.

 
An even better retainer might be to take a washer the same diameter as the seal and weld it to a flat piece of steel and secure it in the same fashion as mine above using the neutral switch mounting bolt. This way the seal is retained all the way around. Some people just make a simple bobbin or spool from nylon and slip it over the pushrod to act against the seal when you pull the clutch lever in but getting the dimension right is crucial for that method to work.
 
It can only happen through ring leakage, even with new rings when they are fitted in wrong position and/or upside down.
The seal moving is also pretty common, too many have been changed without splitting cases so bore is no longer parallel (even the newest 350 is 38yrs old, guys I worked with did them under warranty)
With a 180 crank it won't be pistons, air will transfer between cylinders to a certain extent
Extra breathers are a good thing, fit on into the starter motor opening and one in oil filler, I think the CX 500 extended cap breather fits (been too long since I did this stuff)
 
Hey Free,
I think your retainer looks pretty boss - probably do something similar with both of my CB's.
Casting about in my remaining brain cell, I remember back in my misspent youth, a guy telling me about blowing a seal on his Superhawk. The fix was to use Permatex on the new seal, plus he peened - ie, upset - the metal around the seal with a center punch. Looking back on it, probably the upset acted like little retainers to hold the seal in.
On another but related topic, don't know what the rest of you are using for catch cans, but the big can of Sapporo beer is steel, not aluminum and is thick enough that you can put a decent top on it. I'm using one on the V-twin, got one for the 175 twin and may even use one on the 350. I've got 3 vent tubes coming in (cyl head, former dipstick location and from the former electric starter location) and one going out, to the back of the bike, probably along the rear fender ala the 250 Ducatis of the '60's...
What I hope(d) is that if just one guy / girl reads this post, does this mod to their street ride and doesn't slide down the road on their ass due to a blown seal, then I guess I / we have all done our good work.
Rubber side down, my friends.
Pat
 
We have had seals popping out on 175 racers and on a cb77 race bike. Even with a 180 degree crank and modifications inside the cases to encourage the crank case gas to pas form one side to the other, there's still a pressure build up at high revs.

I fit a large diameter top cover vent tube and a dipstick vent too and run them into an aluminum water bottle from the dollar store.

If a particular motor spits out seals, I have been known to have small tabs welded to the outside of the case to mechanically retain the seal. A small retainer like the one shown might be a smarter idea.
 
It works to hold seal in pace but isn't a cure for excessive blowby
Poweroll pistons are even worse in CB350, the pin offset is greater than stock Honda
Can't remember if reversing piston in bore and cutting valve pocket was much improvement although tightening piston to cylinder clearance then modifying pistons did help to 12,000rpm
Wiseco were also a bad idea until heavily modified, they are just too damn heavy (and also had some major issues with machining before they were all CNC)
 
I just had a cb350 push rod seal blowout just last Friday. Glad a kind person told me about the leak because when I looked down at the light there was about half a qt under me. I got it her home and fixed the seal and also fabbed my own little hold down bracket with washer. Works great but still wondering why the overpressure. I put a small filter at the end of my breather hose. I'm wondering if that's causing restriction for case exhaust venting?
 
Latest thing I've done is fit a spring and washer on gear change shaft to keep seal in place.
The old seal was not tight enough in recess, (it had gone hard from age and heat) rather than mess up crankcase with punch marks I fitted a new seal but left spring and washer as extra precaution
Clutch push rod seal has less surface area to hold it in place, plus,on 360 there is oil pressure behind it (I forget if 350/175/etc have oil feed to transmission?)
 
Like Pat mentioned above, clutch rod seal retainers are mandatory in our club(I like the washer welded on a tab better, but anything is better than nothing), but I too noticed that my shifter seal came loose in my race engine after Barber last fall. I machined an aluminum spacer to fit over the shaft. Even a piece of rubber hose in there would work, as long as it doesn't bind in any way. I also recently added a larger(22mm) hose in the dipstick hole(was 3/8"), and the same size vent hose coming out of the catch can. It looks overkill, but I don't think it'll hurt it! :)

TimV
 
My main concern is, why the over pressurization? Kinda scary to me because I have but one person to call for a pickup. I live in michigan and will not leave my honda by the side of the road.
 
Why the overpressurization? Probably a design shortcoming on the 350 - one little vent pipe in the cam cover for all that volume change going on in the cases plus we constantly twist the throttle a lot harder than (I think) Honda thought we would. Why the pushrod seal is the main culprit - probably the least-retained seal in the cases with the smallest grip area and the pushrod is always moving back and forth through it.
The Honda twins weren't the only ones with the issue; a friend's Triumph twin was one of the great oil pissers of all time until he vented everything he could think of - primary drive case, crankcase, rocker covers, etc etc. Another one of my friends has a crankcase vacuum system on his vintage Triumph race bike for pretty much the same reason. Stock, both of the bikes had vent pipe that bordered on the pathetic, they were so small.
When I started this post, I didn't mean to cause angst among everyone - just wanted to let everyone know about one of the quirks about riding and working on 45+ year-old bikes. The fixes are pretty straightforward and simple enough to do, nor are they costly.
Keep the rubber side down, my friends.
Pat
 
I would suggest doing the inexpensive mods shown in this thread to lessen the chance of leaks. It's an old bike. Shit can happen. Do your best to prevent future problems and ride! 8)

Justin-James said:
My main concern is, why the over pressurization? Kinda scary to me because I have but one person to call for a pickup. I live in michigan and will not leave my honda by the side of the road.
Click to expand...
 
The 350 is a 180 degree twin, there is no volume change in the crankcase at all....Only thing venting is ring bypass....

The later CM models were 360 degree twins, they had volume changes....
 
350 is a 360 twin although I have seen a 180 crank in one
360 is only a 180.
Put a PCV valve in breather line, it does help although you probably need new rings to reduce blowby
 
In a 180 degree motor, as one piston rises, the other falls and that moves gas from under one piston to under the other and that creates a pressure rise and power loss. On a street motor that's not much of an issue and as mid life mentioned, any substantial increase in crankcase pressure is probably worn rings.
 
crazypj said:
350 is a 360 twin although I have seen a 180 crank in one
360 is only a 180.
Put a PCV valve in breather line, it does help although you probably need new rings to reduce blowby
Click to expand...
Pj, r u sure ?
Timing on points is 180 degrees ( 90 at cam). A 360 motor can get away with single points, like the cm series.

The timing marks on the crank of the 350 is the same as 360.




Sent from planet Earth using mysterious electronic devices and Tapatalk
 
I'm just glad to hear that it's a common problem with an easy fix. I haven't had an issue since I put the bracket on. And I took the cheap filter pod off the breather tube and left it open to atmosphere.
 
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