Rev counter gear leak, revisited

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tri-man

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Here's the basics:
73 Commando 850 with new rings, good valves and guides, new valve seals. Rocker spindle flats towards the valves.
The bottom end is good and therefore untouched.
Yamaha XS650 PCV installed on the breather hose, vents to the oil tank.
The bikes goes very well, no smoke or high oil consumption, good compression, fast.
No oil leaks anywhere, except it comes out of the rev counter drive when the revs are kept at 4-5k for a while. New drive housing with seal, new gear with an O-ring on the shaft, and it still leaks.
The breather hose is a loose fit on the stub but still dry as a bisquit, so it would appear that the PCV is working as intended. Besides, why would a pressurized case cause a garter seal to leak? It is designed for that exact job, right?
Little residual oil present when I drain the sump.
So:
Why does it leak from the tacho drive?
What to do to stop it?
 
how tight is the fit of the gear to the housing? if you have an o ring on the gear shaft where is it? if it is at the gear can it push the seal open?
 
You have pretty much the same set up as I do on my 73. I too had the oil leak issue on the tach drive. I finally called Fred at Old Britts and he sold me a modified tach drive housing with a lipped Honda seal. Part# 11-065200/A. It's been oil tight now for 15,000 miles. $35.00 well spent
 
If you have the shaft with O seal, then be careful when passing through the lipped seal in the modified housings, it can flip the seal.
 
The O-ring is at the upper end of the shaft.
I made sure the seal lip wasn't flipped, and to be honest you couldn't flip it if you tried.
Whether the seal is a Honda item or not I couldn't say, but I have a housing machined for a seal, the seal fits well in its housing, and there are no visible damages to it. It's a good and tight fit on the shaft.
I've just fitted a new gear with an O-ring on the shaft, thinking there's no way oil can leak past both a seal and an O-ring.
Apparently it can.
There's always a reason, I might try a different seal, could be a crack in it I can't see?
I was thinking I might have a scavenging issue, loading up the crankcase, but when I stop and drain the sump there's little oil in it. I tried another oil pump, same problem. The oil return seems healthy. There's no restriction in the suction drillings either, I checked when I changed the oil pump.
And if it's loading the cases why doesn't it leak somewhere else too?
If I keep the revs below 4000 I have no issues at all, the whole engine remains dry as a bone. Accelerating through the gears up to 6000 + is no problem either, it happens at sustained highish revs.

Poor oil seal?
Intermittent scavenging issue?
PCV not working well at sustained high speeds?
Too much oil drain from the valve gear?
Have I overlooked something?

I'll test with the breather hose removed.
If the problem remains, I'll try a new seal.
I'll let you know how it goes.
 
Put a new seal in it, everything else seems to be doing what it surpose to in your own words, seals get old they go hard and lose their sealing same as o rings, just because a seal looks good don't mean it is.

Ashley
 
Bill, the shaft to housing fit is as it should be.
Good point Ashley, I will get a new seal even if it looks OK.
 
Hi, what is the size of that seal, I had bought it in the past, but cannot find right now the size , as it's available in most bearing companies , and not only at Honda stores....
 
After 20 years the Honda seal on my tach housing cracked so it was time for a new seal. Apparently the Honda seal is no longer manufactured and the price people are asking for NOS parts is simply ridiculous. For just a few dollars more I was able to buy a brand new tach housing WITH a seal from Andover Norton.

The seal in the new housing required quite a bit of stretch to fit around the tachometer's gear shaft and I was concerned that the seal would be cut by the sharp corners of the slot at the end of the shaft and also by the O-ring groove. I ended up making a 'special tool' which was simply a piece of 1/8 inch thick phenolic material that was sanded to fit into the slot. It was also cut to the same width as the diameter of the shaft. This piece is about 1 1/2 inches long. It was sanded so that it didn't have any sharp edges and I also deburred the slot using a 'Scotch brite' deburring wheel. This 'special tool' I made was simply inserted into the shaft before assembly. After assembly it was pulled out at the end of the housing. As for the O-ring groove I also deburred those sharp corners. After installing the O-ring I buffed it down to about the diameter of the shaft. This serves two purposes: it protects the seal when assembling and once assembled the reduced diameter of the O-ring allows the shaft to be readily lubed externally because the shaft is no longer lubed from the engine's oil. I add a few drops of oil to the shaft every now and then.


Rev counter gear leak, revisited

"Special tool"


Peter Joe
 
The seal I got from my dealer was a 5-15-4 The old seal was 6-15-4 and was worn to ca 6,5. It still felt pretty tight on the shaft.
The new seal was very tight, so I had some molycote on the shaft and inside the housing before assembly. Problem is that the seal and O-ring wipes it off, but I hope it protected the seal at least, and that it keeps the tight seal from instant burn out...
No chance to test it for a while, it's too damn cold out.
 
Revisited part 2
The new seal stopped the tacho drive leak, but as has been pointed out to me, many leaks from the top end mysteriously ends up at the front of the engine. Riding moderately hard, keeping the engine a bit over 4000 rpm, had some oil appearing at more or less the same place. OK, time I started listening then...
Difficult to pinpoint, the leak seemed to stem from the infamous 3rd fin down area.
No I'd already fitted the engine with an XS PCV, neatly tucked away by the battery, venting into the oil tank.
Thinking "it can't be that delicate" I had dismissed it as a factor, but as the options became fewer and fewer and a head removal the next step, I decided to have a new look at it.
I shortened the hose between the stub and PCV to about 4" or 10 cm for the metrically inclined, and let it exhaust through a short hose down between the engine and gearbox.
That actually did the trick. Ran it up to and held 6000 rpm for as long as I dared, several times. Dry!
The owner collected it and rode it home, around 200 km, and not even he could make it leak.
While I can't believe it'll stay completely dry, we called it a success for now, but the long term solution will be to have the head impregnated, or welded if necessary.
By the way, it's and RH4 head, on to which I've used 30 mm carbs and manifolds, and the ports sleeved to 30 mm. The single Mikuni killed performance, it goes like stink with this set-up. Better at any speed, also low speeds, and over 4000 it really takes off.
Only pulled to 6000 in third, but that went real quick. Me and the owner both happy.

To sum it up, a good PCV set-up on these engines is important to keep oil inside, and it's a delicate balance.
 
Thanks for the follow-up, ya.
What drove the decision to vent down, rather than back up to the oil tank?
 
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