« previous next »
Pages: [1]   Go Down

Author Topic: 1975 CB750 Cracked Case  (Read 1823 times)

0 Members and 1 Guest are viewing this topic.

Offline Zeli

  • Full Member
  • *
  • Posts: 10
1975 CB750 Cracked Case
« on: July 28, 2017, 07:40:38 AM »
I am in process of removing the engine from my bike with intentions of rebuilding it. I removed the front sprocket and clearly see where PO has JB welded where the two cases meet.  As I plan on splitting the cases and refreshing the bottom end I just don't feel comfortable having all internals refreshed and welded cases.

My question is which year cases could I use as a direct swap? Taking 75' internals and swapping over to different case
For the crankshaft bearings I'd use the markings from the new case and markings form my 75' crankshaft?

thanks
Logged

Offline lrutt

  • Expert
  • ****
  • Posts: 916
Re: 1975 CB750 Cracked Case
« Reply #1 on: July 28, 2017, 07:48:45 AM »
Just how bad is the crack? was it broken completely out or just cracked. If just cracked, I'd not be afraid to have it welded.
Logged
06 Harley Sporster 1200C, 06 Triumph Scrambler, 01 Ducati Chromo 900, 01 Honda XR650L, 94 Harley Heritage, 88 Honda Hawk GT, 84 Yamaha Virago 1000, 78 Honda 750K w/sidecar, 77 Moto Guzzi Lemans 850, 76 Honda CB750K, 73 Norton 850, 73 Honda Z50, 70 & 65 Honda Trail 90, 70 & 71 Triumph 650s, 65 Honda 305 Dream, 81 Honda 70 Passport, 70 Suzuki T250II, 71 Yamaha 360 RT1B, 77 BMW R75/7, 75 Honda CB550K, 70 Honda CT70

Offline Zeli

  • Full Member
  • *
  • Posts: 10
Re: 1975 CB750 Cracked Case
« Reply #2 on: July 28, 2017, 08:01:44 AM »
Quote from: lrutt on July 28, 2017, 07:48:45 AM
Just how bad is the crack? was it broken completely out or just cracked. If just cracked, I'd not be afraid to have it welded.

Not sure at this time. Probably wont know until cases are split. It goes right over where the cases meet so that wont be fun.
Logged

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 21,100
  • I refuse...
Re: 1975 CB750 Cracked Case
« Reply #3 on: July 28, 2017, 08:49:29 AM »
It's likely a weld repair is quite feasible. If not, Bill Benton lives down the road in Alpharetta, GA and no doubt has full spares (cases or motors). Give him a PM if you opt to replace.
Logged
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Zeli

  • Full Member
  • *
  • Posts: 10
Re: 1975 CB750 Cracked Case
« Reply #4 on: July 28, 2017, 10:40:54 AM »
Heres a picture of what it looks like now.
Logged

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 21,100
  • I refuse...
Re: 1975 CB750 Cracked Case
« Reply #5 on: July 28, 2017, 10:58:51 AM »
Ah, the dreaded chain chew! That's repairable with welding or a case swap or replacement motor.
Logged
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline CycleRanger

  • No comment about being an
  • Really Old Timer ...
  • *******
  • Posts: 5,477
  • Central Texas Shop Manual Advocate
Re: 1975 CB750 Cracked Case
« Reply #6 on: July 28, 2017, 04:50:14 PM »
Hey I've got a motor that looks juuuust like that.  (Only leaking oil from the patch.)
It's sitting under the bench next to a spare lower end.
Someday two shall meet.

If you're going to rebuild it anyway I'd just look for some replacement cases.
Logged
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T

Offline Bankerdanny

  • Eventually I will be old enough in reality to be
  • Really Old Timer ...
  • *******
  • Posts: 7,626
  • Endeavor to persevere
Re: 1975 CB750 Cracked Case
« Reply #7 on: July 29, 2017, 06:08:27 AM »
You'll need to see what's behind that to know. That is a very common place for damage. When a chain breaks it bunches up right there cracking the case and often taking a big chunk out.
Logged
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline The Lone Builder

  • Hot Shot
  • ***
  • Posts: 619
  • "Still running against the wind"
    • Belfast-2-Belfast-By-Bike
Re: 1975 CB750 Cracked Case
« Reply #8 on: July 31, 2017, 05:59:57 AM »
My K2 had a similar issue. I can't remember now how it was originally repaired, but I decided to have it repaired by someone recommended as an expert in alloy welding. Well he fixed the hole OK  although the result was far from  pretty (as the pics show); still I reckoned, you don't see it.

A day or two after putting the cases together with Hondabond (for the 1st time) I noticed a leak of assembly lube. Again I thought, "not visible" so I patched with steel putty. Worked pretty well - no further leak.

However, sticky gear change required that I separate the cases again and that's when I noticed the uneven mating surfaces with some pitting. I smoothed these down before reassembly again with (a bit more) Hondabond and so far no further leaks.

Good luck.
Logged
CB750 K2 - From Belfast-2-Belfast
CB750 K1 - The less, said the better!
CB450 K1 - Stalled.
CB400F Supersport - Not Rusty any more!

Follow my journey through Africa @ http://Belfast2BelfastByBike.com

Offline Zeli

  • Full Member
  • *
  • Posts: 10
Re: 1975 CB750 Cracked Case
« Reply #9 on: August 06, 2017, 03:52:22 PM »
Isplit the cases and it was a pretty nasty crack. I'll upload some pictures tomorrow. Got my replacement cases in. I'm uncertain for the bearings for oil clearances for the crankshaft.   Lettering on new case is different from the original case.... I am guessing I'd use the letter from new case matches to the crankshaft for bearings?  Thanks.
Logged

Offline Jerry Rxman Griffin aka MuthaF'er

  • This MuthaF'er is getting to be a
  • Really Old Timer ...
  • *******
  • Posts: 14,930
  • Bought her new 4/75
Re: 1975 CB750 Cracked Case
« Reply #10 on: August 06, 2017, 03:58:45 PM »
Yes, match new cases with the crank for bearing selection then Plastigauge.
Logged
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)
Pages: [1]   Go Up
« previous next »