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Discussion Starter #1
What's the verdict on refacing cb350 valves? I set up my surface grinder and whirlygig and refaced a set of them, they look perfect, but the more I heard about it, if you take more than a few tenths off the face, you've cut through the stellite coating and the valve will prematurely wear out. Then some other people cited articles about stellite being only on the stems, then others said it was stems and faces, then... on it goes. I'm not above buying a new set of valves, but fuck, if I've got the tools to regrind them, why not?
 

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I'm not going to argue the metallurgy of the valves, but my practice is always to recut the seats and replace the valves. Valves are too cheap and not exactly rare, so there is no reason not to replace them when rebuilding.
 

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Well if someone wants to start a fight, take this, wrap it around a lead pipe, and present it to them up side of their head. The bulletin isn't crystal clear which valves it applies to , but whenever we did heads in the (Honda) shop, we would cut the seats, but if a valve was questionable, it was replaced. I believe the valves that you are referring to fall into this category. Do you have a factory manual? If so I assume it only mentions refacing the seats.

Honda Valves Stellite.jpg

This is from a 1979 CBX factory manual

Valve refacing.jpg
 

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What do they know?....... Oh yes. They are the guys that designed the motor to run for a million miles - or whatever the requirement was back then. So they probably know what they are talking about and for a bike designed to last a lot of miles, that makes sense, but for a race bike or one that is hardly ridden any miles it comes down to how much are valves and how often might you need to replace them? On a race motor, the head is probably coming off once a year or two with very few miles per year. Worth the risk of some wear? Quite possibly. Rebuilding a motor for a client that runs it hard at every AHRMA and WERA etc event all year? The economics might be different.

For my own race motors, I'd cut the valves if they needed it and plan on replacing them every second strip down. For a customer, I might insists they replace the valves because I hate it when a customer says, "let's take the chance" and when it doesn't work out they expect me to eat the work and parts.
 

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Actually thought about including something about cutting them, although it was more geared towards the current "builds" ...if you aren't going to wear the nubbies off the clown shoes, then why worry about how much you cut off the valves. I don't disagree with you about race bikes, but cheap as I am... I still replace valves out of habit. Not trying to apply logic here. I won't cut stellite valves, but reuse some torque to yield rod bolts as in SBC. That make some people foam at the mouth.
I no longer do any work for anyone else so any budget concerns are my own. I have the good fortune? misfortune? of having about 10 years worth of my own shit to do and not sure I have 10 years left where I can physically schlepp motors around. Anyway... folks that cut them generally don't seem to run into difficulties as long as they don't carve off too much. CB350 valves are one thing, but would really be gun shy about cutting DOHC Honda valves such as the CBX. On the car side, they even started grinding cranks which were never intended to be ground for the same reason. If I recall correctly, there was place in Aus that made the bearings, because Honda obviously didn't sell them.
 

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on my m350 twin i went with nos honda parts from the head bgasket up
i did back cut the valves though
if i had not gotten ,my nos head and valves at a yardsale for pennies on the dollar
i wouldn't mess around with the honda stuff why not just put some guides in the head thatare not prone to seizure in extended high rpm runming
use significantly lighter smaller stemmed modern valves
the smaller stems aid slightly bettter flow
Ti retainers aswell. and it shiould enable a bit lighter spring weight and less seat pressure/spring peload
a huge bonus because honda rocker arm folower pads simply have a too thin
hard facing
i have been wanting to do plastic cam lobes
 

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as far as hardness after a regrind just do a before after scratch test
using new swedish chain file like you can get at the stihl saw shop
the are harder than a normal file and very sharp
using your machinssts pro ball player hands gently try to pull a chipo of the valves hard face
after doing a regerind on a valve face that was not pretty
using the same feel ,see if a chip can be curled
 

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Discussion Starter #8
someone on hondatwins suggested sectioning and acid etching a valve, I think that's a cool idea since I have a lot laying around. I found cmsnl or whatever has sets for about $100, I think I'll go that route for now, but it would be good to know the stellite thickness for when cb350 valves become completely unavailable. though, someone will probably figure out which honda car valves are close enough with a little modification and we'll all start using those.
 

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someone on hondatwins suggested sectioning and acid etching a valve...
You don't need to acid etch it once you cross section the embedded piece you just stick it on top of an 80 thousand dollar microscope and measure it optically

... the stellite will probably measure about 7 micron in thickness.
 

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Discussion Starter #12
if you're prepared to pull it apart again and fit new valves when it turns out to be a crap idea, reface them.

if you're not, don't.
yup, if I had a honda engineer who worked there in 1965 tell me to go ahead it's fine, I'd be all about it, it's the not knowing all the information about the manufacture of a part that you're working on that concerns me. I tried scratching the face with an hss endmill and I could scratch it up but I couldn't scratch up the tip, so with a really really basic hardness test, it gave me an idea that whatever surface treatment was on the face isn't there any more.
 

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Do the valves look like they may have been ground before? Probably not easy to tell if it was a light skim and if you don't have a NOS one to compare it too.
You're welcome for the copy of the bulletin btw.
 

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I don't think that CB350 valves were stellite tipped or on the seat faces. Honda, and others used two piece valvee with different head and stem materials. Heads used to be heat treated or coated with something to get that hard skin, but teh valves were OK to grind, even though Honda said don't do it.

The problem with two piece valves is that on a race motor that is revved hard, every so often a head will fall off and punch a hole in other parts. We had that happen on a CB160 years ago. On a street motor, not an issue, on a racer that's ridden hard, fit new valves.
 

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roc that is a good point and the gist of the question
the reason they did the hardfacing in 79 and later is the ban on leaded fuel ewh ?"as you promote the 350 twin is early 60's tech when unleaded fuel was non existent nno ?
 

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aoso if you bakcut the valve @30 degees you can actually bring that angle into the worn cupped area of the valve
effectively narrowing the contact area ,then you are not tricked into grinding the contact area all the way up into the backcut because its nolonger there
on the ex valve especially the contact wants to be pretty close to rhe outer dianeter any way to enable onseat closed time to have the best abjlity to cool down because vthe hottest zone is the outer edge
and that is the only way nthe ex valve can be cooled is on seat time
of course the intake valve wants to have the contact zone nearer to the middle area of the seat angle cut for best flow at low lift time
most of the bused valves i have inspected have not insignificant stem wear anyway and the stem to guide service limit is reached quite often with combined stem and guide clearance it is a bit tricky to measure the stem to guide clearance
but should be checked as layed out in the FSM
i cant remember if FSM mentions this here part but the clearance bmust be checked with clean dry stem and guide
 

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roc that is a good point and the gist of the question
the reason they did the hardfacing in 79 and later is the ban on leaded fuel ewh ?"as you promote the 350 twin is early 60's tech when unleaded fuel was non existent nno ?
I always understood that the changes surrounding lead had more to do with the seats in most cases.... although I am looking at that from my own personal point of view. Most of the stuff I have is from long before lead removal and folks talk about replacing seats, but not much discussion on valves. Usually stainless replacements now and nothing really special. As for Honda, they did it earlier (pre unleaded) to reduce wear.

350 manual.jpg 350 manual 2.jpg
 

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I always understood that the changes surrounding lead had more to do with the seats in most cases
I always believed that lead had little to do with anything other than being a cheap additive.......to bring up the octane rating of poorly made fuels.
 

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valve seat recession used to be a common topic on may forums as leaded fuel became less available. It was rarely an issue with valves - just seats and only for machines not designed for unleaded gas. TEL is of course one the earliest and most used knock inhibiting compounds which all gasoline is laced with because it all has a naturally low resistance to detonation (knock). Then it was replaced with nice clean MTBE which turned out to be worse than lead. Alcohol - ethanol and methanol have much higher knock resistance than gasoline and diesel is way up there.
 

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:I don't see any fights happening here, seems we are all pretty much on the same page.


Did you ever notice how Hydrocarbon fuels including Octane contain exclusively Hydrogen and Carbon molecules, right up until somebody comes along and tries to improve it with an additive, which usually messes it up for either the machinery or the environment :|
 
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