Post by: johnm37 on April 10, 2023, 06:49:57 PM
I got my bike out for it's first ride of the year. It rode for a few yards and stopped. A piece of rag had tangled between the chain and front sproket. Then I made my big mistake. I slackened off the chain and tried to force the rag round the sprocket. I now have a big hole in my gearbox. The damage only seems to be to the bottom part of the casing.
It looks like it's engine out to repair this.
Can I split the gearbox bottom without a complete engine strip?
My repair options seem to be: rebuild the area with epoxy putty. Stick the broken bits back in with epoxy or get the casing welded. That is my favourite. I could get the casing built up with weld or weld the broken bits back in.
What do you guys think?
I'm a capable mechanic type but with little experience on bikes.
The rest of the bike is ok. I don't want to sell it and like the idea of keeping as many original parts as possible.
Thanks in advance
John
Post by: Skoti on April 10, 2023, 07:21:47 PM
Yes you can split the motor without stripping the top end.
You could try a botch up with JB weld or something, nothing to loose.
The proper way is a weld repair which has already been documented some where on this forum.
Good luck
Skoti
Post by: Bryanj on April 10, 2023, 10:09:27 PM
Depending on mileage it could be due exhaust valve guides anyway, they only lasted 15 to 30,000 anyway
Post by: Martin6 on April 10, 2023, 10:55:50 PM
Post by: johnm37 on April 10, 2023, 11:53:54 PM
I'm thinking it's engine out and weld the bits back together. That way I can check the internals at the same time.
I might also look at other things while it's out. I wasn't aware of the valve guide issue. I may check those at the same time.
How do I split the crankcase without removing pistons etc? Getting at the main bearing bolts seems an issue. It sounds like I can check the crank bearings too.
The bike has only done 15,000 fairly careful miles so I'm hoping it won't need much work.
John
Post by: Bryanj on April 11, 2023, 06:26:12 AM
Post by: K2-K6 on April 11, 2023, 08:41:12 AM
What a shame to get caught out with that. Ultimately it needs to be welded I agree with you on that, wouldn't stick the broken bits back in with it in situ. If you want to delay the work then you could make a thin patch in aluminium to cover the hole and epoxy that to the surrounding surface as temporary cover.
To remove the bottom case as noted, I'd also thoroughly clean them all outside firstly to avoid any extraneous grit etc from finding it's way inside as you're not going to strip it out completely.
I like the F2 model and hope you can get it fixed, its not too bad a job to remove that case (motor upside-down) but probably more of a task getting out of the frame as they're fairly close fitted all round.
Plenty of advice on here to help with anything you need to ask.
Post by: martin_uk on April 26, 2023, 11:33:11 AM
Post by: johnm37 on June 10, 2023, 10:46:24 AM
I took the 2 parts along with the broken bits to my welder. He tried to weld it with a variety of rods but they spat everywhere and wouldn't make a run. He was using a TIG welder and is experienced. Does anyone know what sort of rods and gas work for this? Failing that, any recommendations for someone who can weld it? I'm near Abingdon in Oxfordshire.
My final option seems to be a second hand pair of casings. I've found a new pair for sale but at £2700+ they're eye wateringly expensive.
Thanks for the tip about cmsnl. I've downloaded all of their drawings.
Post by: royhall on June 10, 2023, 11:44:41 AM
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Post by: royhall on June 10, 2023, 11:51:33 AM
https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html (https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html)
Post by: McCabe-Thiele (Ted) on June 10, 2023, 11:58:06 AM
Post by: Laverdaroo on June 10, 2023, 12:15:51 PM
I’ve dealt with Zcb-four on numerous times and are super efficient as you’d expect and really good to deal with. Highly recommended.
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Post by: royhall on June 10, 2023, 12:39:50 PM
Try this, bit pricey but may fix your problem.The engine number on these cases looks a bit fishy but it is actually a UK only 750F2. So it is the correct part. Strange how a UK bike has ended up on a German website.
https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html (https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html)
Post by: johnm37 on June 10, 2023, 11:46:42 PM
Yes, DK are good. I've bought from them before. It's quite a distance but could be worth it. They have at least one pair of casings listed. I suspect they will be seeing me soon.
Thanks
Post by: McCabe-Thiele (Ted) on June 11, 2023, 08:40:42 AM
Post by: JamesH on June 11, 2023, 09:48:21 AM
Post by: johnm37 on June 12, 2023, 08:46:36 PM
I've had some good luck on this, I hope. Since TIG welding doesn't work, I decided to try some of these wonder rods that weld aluminium at low temperature. My local stockist (who I tend to trust to sell quality stuff) sells Durafix. Not cheap but worth a try. So far all I've done is tin the part that's broken off. The rods worked really well even at my first attempt. Having put a pool of Durafix on the metal, I ground it back with a power file. It's harder than the aluminium and certainly adhered well. Tomorrow I will try welding the pieces back together and let you know how it goes. Fingers crossed.
There's various video's of Durafix on Youtube.
Also, I have no connection with the company.
John
Post by: royhall on June 12, 2023, 09:18:10 PM
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Post by: Matt_Harrington on June 13, 2023, 09:12:46 AM
Post by: Laverdaroo on June 13, 2023, 09:15:58 AM
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Post by: paul G on June 13, 2023, 03:33:47 PM
Post by: clinto on June 13, 2023, 08:32:33 PM
Took an assistant with oxcy/acet keeping head hot and holding repair peice in position while I held,another torch and filler rod.
Will never attempt again.
Post by: johnm37 on June 13, 2023, 11:44:40 PM
I managed a reasonable repair. It's certainly strong. I hope I've sealed all the cracks to stop oil leaks. I welded the big piece back in. Then I put a piece of stainless steel behind the hole to support the weld puddle. That worked well. The Durafix didn't stick to the stainless allowing me to fill quite a large hole.
I've taken pictures but they're over the 3Mb limit for posting. I will try again tomorrow.
Tomorrow it's the turn of the top casting.
Post by: royhall on June 14, 2023, 06:16:58 AM
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Post by: K2-K6 on June 14, 2023, 07:46:22 AM
A decent coat of something oil resistant may be worthwhile here.
Post by: johnm37 on June 15, 2023, 12:52:59 PM
My next attempt was better. I lifted the engine so the area to be welded was nearly horizontal. That gave a reasonable weld. Certainly strong enough for such a small area on the casing.
Having welded the inside of both cracks, I am going to reinforce the outside with epoxy putty. I use evostick putty. It's 2 parts of putty which you break off and nead together. I've used it before and it sets very hard. It can be sanded, filed, drilled and tapped. It also sticks to almost anything including hands so use gloves when handling it.
There's more room to use this on the outside. This should be the last step in the repair of the casings.
I would attach pictures but they're above the 3Mb limit on this forum.
Post by: johnm37 on June 16, 2023, 08:11:56 PM
I will still reinforce the outside with epoxy putty.
Post by: philward on June 16, 2023, 11:08:39 PM
Post by: Matt_Harrington on June 17, 2023, 09:20:57 AM
Post by: Trigger on June 22, 2023, 09:15:56 AM
It is no good using any type of epoxy fillers as, they always fail in the end. To do this repair correctly the case needs to be bolted down to a inch and a half block of steel to stop any heat distortion from the alloy welding and then welded up a little at a time until it is fully built up, then it is a lot of grinding, milling the matting surface and clean then paint.
You will need to have enough clearance from the final drive cog as, this sits very close to the rear of the cases.