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Discussion Starter #1
Overhauling the head on my 1971 CL175, it only had the seals on the exhaust valves, is this correct, I bought a full gasket set but was surprised it didn't contain these seals, anyone know where I could get some from? Many Thanks Martin. Also do you only replace the little o ring that sits inside the cupped holder?
 

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You do not really need valve guide seals, but if you do want to use them, just put them on the intake valves.
TOOLS
 

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Really confused now, just been on ebay and these seals are being sold for the inlet and exhaust? could someone please put me right! Martin.
 

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That's correct, only the exhaust valves. I also found this suprising, I always thought that inlet valve guides were a cause of an oily exhaust, inlet manifold vacuum drawing oil down the valve guides. Certainly was on 'A' series engines in my misspent youth.

The CNMSL parts listing shows the inlet valve seal as in two parts, but the aftermarket ones are just a single seal.

Confusingly, Wemoto list seals for both inlet AND exhaust, think this is a longstanding error on their part.


https://www.davidsilverspares.co.uk/CB175K6-SUPER-SPORT-1972/part_50250/

https://www.wemoto.com/bikes/honda/cb_175_k6/72-78/picture/valve_stem_seal_inlet

valves.JPG
 

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Discussion Starter #6
Thanks Richard, I have a selection of o rings if I have one the correct size, do you think it would be ok, inside the little cupped holder?
 

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Thanks Richard, I have a selection of o rings if I have one the correct size, do you think it would be ok, inside the little cupped holder?
For the price, I'd go for the exhaust valve seal from Wemoto.
 

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I think Tools reference is to excess valve guide wear on the exhaust side when seals are used, reducing the lube down the guides
 

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If you really want valve guide seals in your engine (maybe to meet emission requirements), only put them on the intake valves.
TOOLS
 

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If you really want valve guide seals in your engine (maybe to meet emission requirements), only put them on the intake valves.
TOOLS
I'm agonising over how to put this question without sounding confrontational or deliberately argumentative, hope you'll bear with me on this.

Honda themselves specify oil seals on the exhaust valves only, which does seem odd to me, as most engines I've been into have seals on both inlets and exhaust. Why did they do it this way ?

The CB200 engine, which is very similar, albeit with different valve guides, has oil seals on both inlet and exhaust. Would the same advice apply to that engine ?

It was my understanding that worn inlet guides were the cause of oil smoke in the exhaust, hence TOOLS comment about emission requirements ?

Worn exhaust valve stems / guides ? Presumably a finding based on experience ?
 

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OK, here it is. In all the Honda twin heads alone that I have done work on, the CB 350 heads do not have valve guide seals and the only time I have had to replace a valve guide was because of physical damage, like a bent valve. The CB 360 uses the same valves as a CB 350 but has valve guide seals on it. Every CB 360 I have worked on has needed new exhaust valve guides. Hence, I have determined that by not having a seal on the exhaust valve guide that the slight lubrication provided to the exhaust valve and guide has enabled them to last much longer.
TOOLS
 

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Thanks, that makes sense.

Would it be fair to say that Honda fitted valve seals on the later engines in the interest of oil control / emissions, with the unintended side effect of increased guide wear ?

One of my 175 heads had a pitted exhaust valve seat, too deep to be lapped out with grinding paste, so I took it to a local bike shop who have a small machine shop, able to rebore cylinders etc. I was hoping they'd be able to recut the seat on their machine, but in the event the chap there did it manually, with some sort of diamond cutter. Anyway, it did the trick, so far as getting a good valve seat was concerned. He also touched up the other three seats and reassembled the head, not that I'd asked him to do this.

In conversation afterwards, he said that in an ideal world, new valves and guides would be fitted, but 'for an old engine like yours, and what you use it for, it's not worth it'. I had to agree, 50 UKP for the work he'd done vs several 100 UKP for valves, guides and fitting.

Engine runs fine, doesn't smoke or use oil. However, as the guides are already slightly worn, I reckon running it without stem seals would let oil too much oil pass through. Matter of balancing short term oil issues with long term further wear, on an engine that's only going to do 2-3000 miles per annum.
 

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Thanks for the explanation Tools. Makes sense but I get Richard's point as well. I'm confronted with the very same question for my 450 project and see an imaginary Yoda above my head, saying: Decision make you must.
 

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Yes, worn valve guide seals allow excess oil down the valve stem and it burns off with the exhaust heat, but badly worn valve guides can cause it too - as well as poor compression if worn badly enough
 
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