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Discussion Starter · #1 ·
Learning about valves today. Got my spring tool and took the valves out of the head. Interesting how it goes together. Going to clean up the valves and measure the springs and clean up the head while they are out. A year ago the most I had done was change the oil. Auto part Carburetor Auto part Automotive engine part
 

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I'm not sure if the CB350 had them, but it may be a good idea to replace valve stem seals (or perhaps fit them even though there are none originally, if possible). Pushrod OHV bikes often get away without them, but OHC bikes usually have more oil pressure up there, and if the stems start leaking, the engine may start to burn oil.
 

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That’s an interesting point about the difference between OHV and OHC engines that I hadn’t appreciated before; I thought that they
(OHV) only need valve stem seals on the inlet valves because during the inlet stroke the depression in the combustion chamber would draw oil down the stems of the inlet valves but the exhaust valve guides wouldn’t leak because the ex. Valves were closed, and when they were open the gas flow would be pushing oil back up the stems.
There you go!
Nigel.
 

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Discussion Starter · #4 ·
This head did not have any valve stem seals but when I look at my gasket kit I think I see them.
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That’s an interesting point about the difference between OHV and OHC engines that I hadn’t appreciated before; I thought that they
(OHV) only need valve stem seals on the inlet valves because during the inlet stroke the depression in the combustion chamber would draw oil down the stems of the inlet valves but the exhaust valve guides wouldn’t leak because the ex. Valves were closed, and when they were open the gas flow would be pushing oil back up the stems.
There you go!
Nigel.
I think since the 70's, these seals were fairly standard on all Japanese motorcycles (I think even the OHV Honda CX models had them). OHC almost always run the cam in plain bushings and need lots of oil pressure up there for it (so a lot of oil).

Meanwhile, BMW airheads never had valve stem seals. There is oil pressure to the head, but it only lubricates the rocker arms and a little on the pushrod tips and between the rocker and the valve (hot oil mist lubricates that either way). I know Moto Guzzi did not use them either for a very long time, I doubt even the 2012 California had them (essentially the last classic 2 valve Guzzi engine, used since the 60's, with various improvements through the years). Also, Moto Morini 350 motorcycles had absolutely no pressurised lubrication of the heads and were good to 9k rpm. Lubrication was when hot oil mist got onto the valve cover, condensed, and started dripping down onto the rockers on these spikes above the rockers, that were cast in the valve covers. Small block Guzzis (V35, V50) used the same design, but also had a tiny hole from the oil gallery that helped lubricate the rockers a bit more. I figure a little bit of oil in the valve stem is really not that big of a deal - it should make it last even a bit longer. But the problem is that once it gets too worn, these engines will start to consume oil (well, except Morini which runs almost dry up there anyway). Ducati, on the other hand, used to use some very odd seals there (essentially an o-ring inside an inner slot in the valve guide) on Pantah models, but later adopted standard valve seals. They were OHC, after all, and had lots of oil pressure in the head (even though I think their cams were supported on roller/ball bearings, not plain ones, and those require less oil pressure - but those fancy desmodromics probably need lots of oil to prevent too much wear).

All in all, the way the Japanese did it prevailed, and it is quite fortunate such solutions are taken for granted on these old bikes, while they were still completely alien to some European manufacturers just a few years ago.
 

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I’ve had BSA and Triumphs mostly; and one Ariel Arrow 2 stroke, I never worried about a little oil on the valve stems; indeed it didn’t seem to matter, the guides were bronze and seemed to be self lubricating to some degree. I just never gave it a thought! It worked, why try to fix if it ain’t broke?
You have an extensive knowledge of these exotic bikes! I just ride them. I’m working on a 500/4 k2 at the moment and just about to do the head and fit the valves; I’ve read you don’t need to lap the valve seats because doing that will remove the Stellite coating. What’s your view on that? Safe to lap or recut the valve seat with the valve seat cutters?
Igel
 

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Well, living close to Italy (I'm from Slovenia), these bikes are as common as Japanese bikes. Never owned or ridden a British bike, though (well, I have a 31' OHV JAP 350 engine and a BSA M20 4 speed gearbox that I'd really love to fit on some nice speedway-ish frame, but that project is still very far).

If you recut the valve seats, perhaps lapping may not be needed. I'd try assembling it and pouring some fuel into the port. If you see it leaking out through the valve into the combustion chamber, it will need some lapping. Sometimes a bit of dirt may get on the seat when you assemble it, and it will keep it slightly open, so I tap the valve with a plastic hammer a few times to make it sit nicely before doing this test.
Personally, I've always lapped the valves if they failed the fuel-in-the-port test. Got a 100 years old lapping machine for it that makes this job really easy (looks like a hand drill, but turns back and forth). No coating will have any real benefit, if the valve surface is so bad that it leaks...

I recently had a similar problem - I was rebuilding a Moto Morini engine, and the crank needed to be reground for undersized plain bearings. Originally, they surface hardened the crank by nitriding, and grinding it probably already removes that layer. But then I thought - if it was already worn, and if the nitrided layer is so thin, then that layer is already gone in most places either way. Since the manual suggest the factory made up to 3 undersize bearings, I think I should be fine if I only used the first undersize.
 

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That’s Old School Pikl and good to hear your thoughts. Lapping seems to be a three part process, coarse; then fine grinding paste and finishing with a polish like Solvol Autosol. I’ll assemble first; leak down test and then decide what needs doing.
You have an engine and a gearbox you just need to fabricate the front and rear engine plates in 1/4 duralumin and then you can fab up a frame with a head stock with the appropriate geometry, use a Honda TLS front end and depending on the gearbox orientation use a hard tail rear end and you’ve got a useable bike.
Love a challenge!
Nigel
 
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