750 F2 chain damage to gearbox

[johnm37]

Hi to everyone. I'm a new member having only just dicovered this forum. I don't like starting by asking for help but I've a problem.
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

[Skoti]

Welcome to the forum John,
 
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

[Bryanj]

What you could do ismeticulously clen and epoxy the bits in so you can use then have it welded when the engine needs work.
Depending on mileage it could be due exhaust valve guides anyway, they only lasted 15 to 30,000 anyway

[Martin6]

Feel your pain. Are you certain nothing has gone inside the case?

[johnm37]

I'm fairly sure I've got all the bits out. I've looked at an epoxy repair. It's a possibilty but supporting the broken parts while the epoxy sets isn't easy and there's not much clearance between the case and gears.
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

[Bryanj]

All te side cases come off then you unbolt all the case half bolts and remove the lower case

[K2-K6]

Welcome to the forum John.

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.

[martin_uk]

If you dont have a parts book to see which parts split worth checking https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/partslist/

[johnm37]

Well I've finally got the engine out and split. The bad news is there's the hole in the lower casting but the top casting us also cracked in 2 places.
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.

[royhall]

Spitting like that is usually contamination, are you sure they were really clean and the black paint removed. They also need to be preheated and welded a bit at a time to avoid distortion. Give DK in Stafford a ring, they may have some good used cases for sale or try good old eBay.

Sent from my Pixel 7 Pro using Tapatalk

[royhall]

Try this, bit pricey but may fix your problem.
 https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html

[McCabe-Thiele (Ted)]

Might take some time finding one but a good used case looks like the best way forward.

[Laverdaroo]

Cheaper than the ones found already and would suit if correct

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.


Sent from my iPhone using Tapatalk

[royhall]

Try this, bit pricey but may fix your problem.
 https://www.cb-four.com/cgi-bin/shop/__Engine-Block-Honda-CB750Four-F2-G-used-__21-3297-10_07.07.html

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.

[johnm37]

My welder did clean off all the paint and also pre-heated it. He only tried on a broken piece but he doesn't think it's weldable.
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