Topic: Hole in Crankcase

[PFC Rairden]

i have a 74 cb750, recently the chain snapped and broke through my crankcase, the issue i have is weathor or not to try and find a decent engine for cheap(starting to be hard to find) or try and replace the lower engine, i honestly dont even know where to start, so if anyone had any suggestions that would be great

[Roach]

can you take a picture so we can see how bad it is some people have had good success with getting a shop to weld the hole up.

[754]

 Where in the World are you...

[PFC Rairden]

i actually cant take a pic right now but im workin on it, the hole is next to the gear where the chain spins, what i was thinkin was just tearing down the whole engine cleaning and polishing everything then seeing about getting it welded, but i dont know if i biting more than i can chew, i live in gatlinburg tn and i have a few months to get ready for riding season so i want to make sure everything is right, i was debating on puttin a hard tail kit on it with a springer solo seat but money wise it prob best i stay with the original frame until i can save some money and do everything right

[Gaither]

Gatlinburg - How much snow did you get last week? 'Bet the mountains were beautiful (and impossible).

Chattanooga got 6 to 8 inches. I hate the stuff.

[Stev-o]

That was a common problem with the early CB's.

Personally, I would look for a used motor to slap in.
They are out there, same basic motor was used from '70 - '78.

Good Luck.

[Jerry Rxman Griffin aka MuthaF'er]

Gatlinburg, eh?! My vote is to weld it and join up with the 2011 Relay Rally Through Tennessee.

PM me and I'll add you to the list. We need E. TN and E. KY riders

[wohali]

Post a pic for sure. I have similar damage to my crankcase but it's above the seam between the two halves - and the shop thinks they can weld it up and possibly apply a bit of epoxy to make the surface nice and smooth. If you choose to repair it, keep an eye out for oil leakage after the fact and be prepared to take the engine back to the shop for re-patching if it leaks!

Reminder: this isn't something you want to try welding yourself unless you really know your way around a TIG welder and working on aluminum alloy.

[Shoeboxjoe]

I found a similar crack shortly after starting my cafe project. Luckily i have a friend who is a welder and does a lot of aluminum. He said it was a huge pain in the ass but it turned out good in my opinion. Had to put a little epoxy to seal it up tight still. Here's some before and after pics...

Before


After (Top case upside down obviously)


Obviously the engine needed to be completely torn apart for this repair. The case was pushed up against the transmission which shaved off pieces of aluminum that had to be cleaned out. All in all it was good experience and im glad i did it.

[faux fiddy]

I found a similar crack shortly after starting my cafe project. Luckily i have a friend who is a welder and does a lot of aluminum. He said it was a huge pain in the ass but it turned out good in my opinion. Had to put a little epoxy to seal it up tight still. Here's some before and after pics...

Before


After (Top case upside down obviously)


Obviously the engine needed to be completely torn apart for this repair. The case was pushed up against the transmission which shaved off pieces of aluminum that had to be cleaned out. All in all it was good experience and im glad i did it.

In 75 years they will all have this repair.

[daytonajoker]

There's already a discussion on this, I worked in a Honda shop in the early 70's and we used Marinetex to patch the hole, recently I picked up a 750 and used JB Weld....just drain the oil, clean the area real good with brake cleaner, scuff it up and paste on the JB,put on a patch of fiberglass and more JB....several layers of glass and JB and it'll be good as new....grind it down for chain clearance and put the side cover back on....only you will know it's there and if you eventually have to split the cases, cut it with a die grinder

[HedNut]

Had the same issue with mine..but a chunk was broken...the po patched it with some sort of epoxy and mashed the broken piece against the gear inside.... Dumbtat!   I tore it down and had it welded...cost me $60 at a shop here.  Wasn't pretty...but it worked!  I used a bit of JB where the top and bottom cases mate to ensure it had a good surface...

[BAchvytrk]

I have a used engine, the numbers come back as a 74, that i just got with a chop project i bought. the top end is disassembled, would sell the bottom end alone, or the whole motor, fairly cheap if you want to go that route.

[PFC Rairden]

ill try and get a pic up as soon a possible but the hole is a lil smaller than a dollar, so what my dad and i did is take a piece of aluminum(really a cambells soup can) and cut to fit with some jb weld to give it a tight bond, being from TN we had to use some redneck engineering lol, the question now is, in the crankcase by the front sprocket where the hole is, it doesnt hold pressure but just the oil itself, right?

[Roach]

doing the soup can  will only be a temporary fix after a while the vibrations will crack the jb weld and you will be back to square 1. it is winter i would do a tear down and get it fixed before riding season starts. you dont know if there might be some metal floating around inside the cases.

[MCRider]

Plenty of cases around, I've got a set too. If you restrict yourself to gaskets and a case swap, you can do it in  a weekend. You might need different main bearings, that would add a week or two. Possibly not, if you're not too picky.

[Stev-o]

Buy the motor from chevy trk!

[PFC Rairden]

right now its slow season here and this town becomes a ghost town, the economy is strictly based on tourism, and i figured the soup can wouldnt be the best salution but right now its free and if it works it works if it doesnt then im gonna have to buy an engine anyway, how much did u want for the engine? and where are u located?

[BAchvytrk]

i'll sell the engine for 200 plus what ever it is to ship. i'm in buffalo, ny

[CrankyOldGuy]

When my chain broke in the early 70's and put a hole in the crankcase, the dealer replaced the crankcase and Honda provided a small chain guard to put around the front sprocket.  Does the guard actually work in preventing the free end of the chain whipping over the front sprocket and impacting the crank case?  ... or was it mainly to pacify us users into a false sense of security? 

Harry O

[MCRider]

When my chain broke in the early 70's and put a hole in the crankcase, the dealer replaced the crankcase and Honda provided a small chain guard to put around the front sprocket.  Does the guard actually work in preventing the free end of the chain whipping over the front sprocket and impacting the crank case?  ... or was it mainly to pacify us users into a false sense of security? 

Harry O

The auxillary chain guard was the subject of a thread not too long ago. I'd seen them when we warranteed cases back in 70/71. I think as do many it was simply a knee jerk reaction "we have to do something". And it looked good. Never saw a chain break on a bike with the extra guard, so who can say?  In any event it was at best a bandaid. The problem lay elsewhere and was fixed, elsewhere.