Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Johnny4428 on January 23, 2021, 02:34:21 PM
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I had a nice simple job to do this afternoon, splitting the 750 crankcases. By the time I cleaned the lairs of chain grease from the back of the engine I got a dirty drop, this bike has suffered at some time from a broken drive chain.
It’s going to be fun trying to take these apart!
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I have seen loads of them in my workshop :o
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I have seen loads of them in my workshop :o
And you've done some very nice tidy repair jobs on them!
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Has that been welded across the crankcase joins?
My eyes could be deceiving me here.....but I have seen some oddball stuff done that defies logic.
I guess the guy just wanted a quick fix to stop the oil falling out, and didn;t want to take the motor out of the frame and go to the trouble and expense of having it done right.
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Yes the repair has been done with the engine in one piece. The need to get in the inside has just become a lot greater. Maybe small diameter cutting disc to get the thing opened up, following the line of crankcase seams. Bolt broken in the sump as well flush with crankcase but managed to remove with small pin punch and hammer.
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That's a bummer Johnny,
I don't think you need to be too fussy about getting in with a thin disc as both cases will need work anyway. Graham will no doubt have thoughts on the best way but I would be inclined to chain drill a series of small holes across where the original joint line should be. Either way, the output shaft on the box is getting in the way. You may need to put that rod nearest the damage halfway down on the exhaust or power stroke so it's out of harms way when you break through.
David Henning might have some suitably shaped pieces of casing to fit the gaps. I'm not sure if Frankie Bain is still doing alloy welding - he's the best man up here, as you probably know.
Also in the second pic, is there a half broken hole?
Ian
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Thanks Ian for advice. Yes the rear lug (broken) catches the case that goes over the drive sprocket, I suspect the damage was probably all done together, would explain why there was no sprocket cover with the bike. Haven’t seen Frankie Bain for a while, I knew him quite well from Flotta working days. There’s another guy Kevin Tait, think he is up in Sandwick somewhere also welder in Flotta.
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I have done quite a few like that, some that look the same, some that had a plate over the hole with the plate held on with screws that went through to the cogs and some that have had a lot bigger hole than that. I even had one that was welded up and they had not removed all the old broken bits from the inside :o
The way forwards is a dremel cutting disc which is the easy part. The hard part is getting it welded by someone who knows about heat distortion on this type of alloy. The last one i did, the hole was that big, i had to make a jig up from a 27mm plate to stop distortion from the heat. Weld for 15 seconds and leave to cool for an hour. And when you have spent more days on it than you can remember, you have to mill it flat for a good old Honda mating surface ;)
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Thanks Trigger, I’m amused about the plate with the screws. I was thinking this repair was a bit industrial, although effective. I will post progress (good or bad) on the forum in the coming days and weeks.
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Quite surprised the alloy has not bent in and been catching the mechanisms behind it. There's next to no room at all behind there.
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Quite surprised the alloy has not bent in and been catching the mechanisms behind it. There's next to no room at all behind there.
That's what happen with the engine that Graham mentioned Roy. The engine would rotate forward, then get jammed, it would rotate backwards and also then get jammed. It was where the numpy that did the original repair had screwed through with what looked like a wood screw and it was fouling on a cog 🙄🙄🙄🙄
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Quite surprised the alloy has not bent in and been catching the mechanisms behind it. There's next to no room at all behind there.
That's what happen with the engine that Graham mentioned Roy. The engine would rotate forward, then get jammed, it would rotate backwards and also then get jammed. It was where the numpy that did the original repair had screwed through with what looked like a wood screw and it was fouling on a cog 🙄🙄🙄🙄
Always the same story with restorations. You spend 10% of your work fixing the ravages of time and 90% of your work fixing the damage caused by the shaved apes that previously owned it. ;D
I think I would be looking for a set of replacement cases rather than spend endless hours on that. You can butcher them open then to save the contents. Plus you know it's right on re-assembly.
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Most of the cases i have repaired have been sandcastles and very early K0. You should easy find a set of K3 cases ;)
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How did you get the sand-cast finish back Trig. Artistry at its best.
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That was the easy bit, getting in! No further damage.
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looks better inside than I thought it would do.
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Good job so far!
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Good luck with the repair, At least you know its possible.
Had similar with a K1 750, working on the motor and found a load of epoxy resin around the front of the sprocket. Damage where the chain had popped in for a visit. I was very fortunate as a mate had a bit of Kawasaki case that could be made to fit and had it welded up by an FIA approved aircraft welded. Only cost me a case of beer too.
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Yes very annoying to discover the damage. I need to speak to a “metal smith” now and arrange repair. If the worst comes to the worst I know where to source replacement cases but would like to have these repaired.
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If you are going for a weld repair, make sure the gear box is in place as, some of the cogs are very close to the case ;)
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Crankshaft looks good within spec. Centre bearing shell looks iffy!
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Can't believe your going to weld that. Plenty of good K3 cases out there.
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I hear you Roy! That is just the kind of comments I need to ensure I do the right thing. Cheers.