Topic: Chain broke & cracked my engine...need some serious help here!

[jdubb1977]

I posted a week or so back that my chain had broken (1976 CB750F).  well, i pulled the front drive cover this weekend and was horrified to see some serious damage to my engine case.  as you can see from the pics, theres a smallish hole.  well, the triangle shaped piece btwn that hole and the engine seam is completely broken and lose as well (making it a very large hole), and there is a crack running up and down from that area on the right side. 

i've done some reading on people patching w/ aluminum and RTV or jb weld, but this seems like it's too big for that.  can it be salvaged?  welded?  other options?i really can't stand the thought of scrapping the engine. 

[ekpent]

Bummer- was hoping that your chain just gracefully ran out of there. Only options really are cleaning well, scuffing up and slather in the JB, find a good welder who may be able to save it or replace the cases    That one does look pretty severe. What year/model is that ?

[70CB750]

Wasn't it 75F what he has?

I would look for new engine.

[ekpent]

I recently acquired a 75 or 76 F torn down engine I plan on parting down the rest of the way.

[calj737]

To fix it properly, you need to have it welded. To weld it properly, you need to tear it down. If you are tearing it down, it may prove to be more desirable to either replace the cases, install a donor lower end, or replace the motor.

The reason I say tear it down to weld properly is, you need to get proper saturation of the weld to hold in that area, and there is a strong likelihood of weld purging through to the backside, and that needs to be ground smooth for zero clearance issues.

I would tell you to expect to pay $300 for a quality, competent weld repair of that. And that would be with you tearing down the cases and rebuilding it.

[70CB750]

The halves were machined together, replacing just the bottom is not enough.

[Bodi]

This was a severe problem with the 750 engines: Honda replaced A LOT of engine case sets under warranty because of it, finally blamed it on the battery vent dripping acid onto and destroying the chain.
Out of warranty repair tended to be welding or otherwise plugging the hole: replacement cases cost a bundle plus a shipload of work moving the rest of the engine over.
You have to take the cases apart to do a proper welding job but it can be done with the cases together. There are lots of ugly looking from-the-outside JB Weld jobs driving around. It's worth a try, make sure the metal is super clean before gluing it. Look for any broken off bits in the sump: they should be caught by the oil filter screen but it's worth removing any debris.
There were steel "case protector" things made that were supposed to save the case if a chain broke. I've heard they didn't work, the case still broke and sometimes the sprocket shaft broke the case around its bearing because the force was so much higher with the steel plate in there. There's just no room between the sprocket and case to get a seriously strong steel protector in there.
Use a modern chain (a decent one will be many times stronger than a 70s chain was) and make sure the battery vent is nowhere near the chain is.

[calj737]

The halves were machined together, replacing just the bottom is not enough.

Lower end is referring to the motor below the jugs in this instance.

[jdubb1977]

@ekpent:  me too!  what a mess this has become.  i may go the JB route as a first try.  if it doesn't hold, any idea what you'd want for your cases? wanna trade.

[jdubb1977]

thx guys.  this is seriously such a huge pain.  and thx Bodi for your tips.  gonna take 'em!

if I go the JB route, I'm guessing I'll need to remove that section of the case above the hole (actually to the left in my pics), since it's essentially floating loose there.  that'll make quite a huge hole.  what do you recommend filling/covering that with before applying the JB weld?  wire mesh?  thin metal?  and should i apply the JB to the existing seam btwn the cases just to make sure there are no leaks afterwards? 

[750K]

I'd start looking for a donor motor if it were me, that's brutal looking...

[Nic]

I've seen much worse than that and still managed to do the bodgy fix, you're really only worried about oil leaks, we used to cut up a steel ( in the good ol days ) coke can and make a patch, clean the area, remove all the loose bits, rough it up with a drill and wire wheel and just epoxied first on the cases, then fit the patch and epoxy over the patch so you have a good thick seal around the patch. This kind of repair worked well and if it leaked you simply apply more epoxy. I would never freak out and look for new engine or cases, just do the patch fix IMO n yours isn't that bad compared to others I've fixed. Always a good idea to drop the sump n look for loose bits of case too but while it initially looks real bad, once you gum it up n stop the oil leaking you just get back on n ride. Take the sprocket cover off n check periodically. It could always happen again down the road even with a new set of cases.

[Yoshimatic]

Nic has the right idea. I had a hole way bigger than that back in the seventies from a thrown chain and I just cleaned it up smeared a healthy amount of Devcon on it and a healthy amount of Devcon on a piece of Milo tin cut to shape. Pushed the tin over the hole with slight twists and let it set. It never leaked or played up again.
Devcon is a plastic steel putty but almost any epoxy stuff would do. Cleaning up the cases with Electra clean or even acrylic paint thinners is best.
Any large tin from baked beans would even do once it's cut open and cut to shape and bent to shape.
Remove all broken pieces that you can first so pieces can fall inside.

[ekpent]

@ekpent:  me too!  what a mess this has become.  i may go the JB route as a first try.  if it doesn't hold, any idea what you'd want for your cases? wanna trade.

Definitely give the patch method a try, many have been done over the years (something to check when buying a bike) with success. If your interested someday in the cases will try and make them reasonable but even actual shipping out to CA can get a little expensive.

[jdubb1977]

i might have a welder look at it first just in case, but you guys might have convinced me to give the patch method a try.  really happy to hear that this looks fixable.  so based on what you've said, a modern day coffee can and a healthy amount of JB weld should be strong enough, yeah? 
i pulled the oil pan.  there are a few shavings in there, but nothing major.  any other way to check for that missing piece of my engine case?  would using the kickstart to rotate things possible push something out the bottom while the pan is off? 

@ekpent, thx for the offer.  if things down't work out w/ these other methods i'll definitely be in touch!

[Bodi]

I would want to find that part, if it gets into the gears you would have a problem. Try squirting oil or solvent in the hole - it may be just sitting below there stuck with oil or crud.

[bjbuchanan]

one of my engines has the a fat patch on it too, you will find a bunch of these bikes with them like that now. The chain grime does a good job of hiding the patch when the PO tells you it is a mint case.

Make sure you get all the crud out and patch it.

[jdubb1977]

so the next hiccup in this saga.  that triangular piece that's totally lose in the pics won't come out.  I can really only get one angle on it, and i've tried prying it, and even took a dremel to the edges for a while (yep, i'll be changing my oil very frequently once this is done).  it won't budge.  gotta get that thing out before I can do and patching, as its resting against the gears on the inside.  any ideas?

[Stev-o]

I'd start looking for a donor motor if it were me, that's brutal looking...

+1

[Retro Rocket]

I'd start looking for a donor motor if it were me, that's brutal looking...

+1

And buy good chains and keep them maintained...

[lucky]

The 77-78 models had a thicker case in way of the chain.

Some had that massive #630 chain that really packs a punch.

I cannot really understand that photo. I wish it was a larger view.
Is it the sprocket cover??? IS the photo right side up?

It can be TIG welded though,
but has to be taken apart.
Has to be clean and free of oil.

[Yoshimatic]

Try a baby slide hammer to get the loose piece out or maybe a couple self tapping screws in it and pull it with vice grips.

[lrutt]

I'd have to ask what made the chain throw? did it actually break? did the MC come off? If it broke, what were you doing? drag launches or just riding? Must have either been an original or one cheap POS chain.

I have to repeat that a chain is NOT the place to try and save a few bucks. Buy a GOOD chain. I always look for the highest tensile strength I can get.

You had better pull the pan off the motor and see if you can get the other chunks out the bottom as well. Just to be safe. But I'd do the patch, what can it hurt, and likely will last your lifetime.

[ekpent]

I'd have to ask what made the chain throw? did it actually break? did the MC come off? If it broke, what were you doing? drag launches or just riding? Must have either been an original or one cheap POS chain.

I have to repeat that a chain is NOT the place to try and save a few bucks. Buy a GOOD chain. I always look for the highest tensile strength I can get.

You had better pull the pan off the motor and see if you can get the other chunks out the bottom as well. Just to be safe. But I'd do the patch, what can it hurt, and likely will last your lifetime.

His master simply link fell off/out and spit the chain off. There is another thread that maybe the OP can post a link to.

[jdubb1977]

here's a link to the other thread.  http://forums.sohc4.net/index.php?topic=140241.0

i put a low end RK chain on there.  it came loose 300mi later and here we are.  still scratching my head on how to get that loose piece out.  it's a good 1/4" thick at the top, and there's no way it's coming out.  i'm considering trying to adhere it to the patch so it pulls it away from the gears.  if i end up pulling the engine for all of this, i'll probably end up getting OCD and rebuilding the engine.  i'd rather be riding!!