SOHC.co.uk Forums > CB750
Rewiring K2 with later high beam switch
K2-K6:
I've never used anything but Blue Hylomar on these motors for all flat gasket surfaces and it always works.
Clean the surfaces with petroleum based cleaner, apply thin film of BH to entire surface, no excess though and let it dry for 15mins or so before assembly. If there are variance in flatness then coat both surfaces.
Don't know if you're using silicone? but I rarely use it on any engine as I just see it doesn't match the application unless specifically requested or the surfaces have such a poor alignment to need big gaps filling in.
Those Honda castings are usually quite good and I've not worked on one that I consider would need that amount of sealant.
This may be related to your oil flow problem, but I've often found silicone gasket material inside motors when stripping them as most of it squeezes out of the mating surfaces. It's not good, and once dismantled a Kawasaki Z900 that had an oil pick-up that looked like a pizza there was so much silicone on it.
With any cams I've always used a very fine graphited grease on all bearings and lobes to prevent pickup on any initial turnover, you don't need much but just enough to coat lightly each surface.
the-chauffeur:
I'll be using ThreeBond on the case halves. I generally use acetone to wipe down the mating surfaces and as far as I can tell, they're properly flat and not flawed. Which is a little more than I can say for some of the various sidecovers. I've been beset with minor leaks from those, too, and so this time I've taped some fine-ish wet 'n' dry (600-1000 grit) to a flat board and given the mating surfaces a rub over. That process revealed a number of high spots, which are no more - hopefully that'll put paid to the minor seeps.
But believe me when I say I'm having a run of extraordinarily bad luck with bikes right now, which includes:
- two water leaks from somewhere in the front outer case surfaces of my ST1100 (main transport). Looks like they are around the timing belt cover - the water pump housing sits behind the timing belt, so one of them may well be a rubber seal in that assembly; the other seems to be from somewhere around the front clutch cover, which makes much less sense. Whatever, it's a job that I have no desire to tackle myself right now (with the 750 already in bits) and is likely to be so expensive, it might be more than the bike's worth
- whilst admiring my newly powdercoated SL350 frame that I collected today, I've noticed a dirty great crack in the RHS swingarm spindle mount. I thought I'd got all of the anomolies on that frame sorted before I got it coated, but I guess not. Yes, it's structural so no, I can't simply patch it. I gotta get the powdercoat off and get it welded, then re-powdered.
If I didn't have bad luck, I guess I'd have no luck at all. Suffice it to say this really ain't my year.
the-chauffeur:
I'm back . . .
I can't quite believe this. The 750 cases are leaking - again - from the same place as last time; the joint that runs across the part of the casing that separates the gearbox from the drive sprocket. Everywhere else on the crankcase joint is solid - no leaks from the front or other sides. As before, it's a tiny, tiny hole, but just enough so that as the crankcase pressure builds it lets out a small stream of oil. I really, seriously cannot understand why the leak has appeared - I checked and double-checked this area to make sure it was flat because of the last leak, and trowelled on the sealant in that area. Frankly, I had some concerns that I might have overdone it sealant-wise there, but nope - seems not.
Two observations: the lower casing has (very briefly) come into contact with a drive chain at some point in the long-distant past and there are a couple of shallow channels that are the chain width apart that run about 1cm across the join. And although I'm pretty damn sure the cases are the original matched pair, the lower case edge protrudes few fractions of a mm past the edge of the top. The leak is coming from somewhere between the channels, but the hole is soooo small, I'd need a microscope to see it (it's one of those where the oil doesn't stream out - it just kind of accumulates along the tiny lip and then runs off.
I've drained the oil and cleaned the sides up with white spirit and a cotton bud dipped in acetone. I've also run a couple of sharp tools along the area of the join to tidy it up. Bearing in mind the lengths I've gone to so far, and the lack of success I've had with this particular area, what I plan to do - and I need some guidance here - is to completely clean the area around the leak, (carefully) dry it out with a heat gun and then run a thin bead of JB Weld (metal epoxy) along the join. The US board is full of stories of folks who've used it to fill in/rebuild broken cases, so it seems the least destructive/best plan of action right now. Gasket sealant clearly isn't doing it, so I figure that before I pull the engine again I've got to try everything I can to try to cure that bloody leak - and carefully applying with something more substantial may be the answer.
On the subject of leaks, I think I've identified why my side cases have been leaking. After flatting them off and refitting them, I've found that no matter what type of gasket I've used (getting progressively thicker) they're still seeping - more from the front left than the rear left. But it dawned on me today that the leaks are from around the fasteners that fit inside locating dowels. What I guess is happening is that the side cases are being held very slightly proud of the engine faces by the dowels, and so even though I've spaced the sides sufficiently with thicker gaskets, they're not taking up the space the dowels create. So I s'pose I gotta shave the dowels down very slightly . . .
Thoughts on the joint issue very gratefully received . . .
K2-K6:
I've no experience of JB weld so can't offer anything contructive or otherwise on that score, just some other general comments.
Fixing a leak from outside is usually marginal in almost any situation you can think of especially if pressure is high. This should be a low pressure location so odds are a bit better and with really good cleaning you may stop what appears to be almost a capiliary action, so on balance worth a considered effort.
Things like isopon filler stick really well to clean alloy and if not suitable can be very carefully filed away so very low risk.
Are you confident of any breather cleaning to help avoid crankcase pressure?
The dowels do have to be checked if you've reduced surafce level at all, bolt on without gasket and check with feeler gauge to get accurate read on it.
the-chauffeur:
Thanks K2.
Capillary leak sounds about right, and I know exactly what you're saying about external seals. I've been wondering about maybe v-e-r-y carefully filing/Dremel-ing a V shape into the join faces to create greater surface area for the epoxy to stick to, but decided to go with the flat for now on the basis that doing it this way is less invasive/more easily reversible.
Not quite sure what you mean about the breather but the pipe is new and doesn't have any snags or bends in it. I have, however, got a question about where the breather should join - something I've never properly understood. On my K2, the oil tank has 2 connections on the rear - one high up for a larger pipe, one lower for a narrower pipe. Right now, I've got the breather pipe from the cam cover going to the top connection on the oil tank; I've also got a small pipe running from the small lower connection to a similar size connector on the rear of the crankcases. But I don't know if that's right. Should it be closed circuit or should there be a breather-to-air? And if there is a breather-to-air, what are the connectors on the rear of the oil tank for?
And thanks for the pointer on the dowels - feelers are the way to go.
Cheers
I'm sure I've asked for advice on this before
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