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Offline Rydog400f

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Bent connecting rod?
« on: June 01, 2019, 09:22:44 PM »
Pulled open my project cb750f and noticed one of the connecting rods looks bent? No sign of damage on the jug, piston or head. Is this normal or should I replace this?
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Offline 754

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Re: Bent connecting rod?
« Reply #1 on: June 01, 2019, 09:53:22 PM »
If it was running like that you will see a strange pattern on the piston.
 How did the dowel get bent ? It need changing.
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Maker of the WELDLESS 750 Frame Kit
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Offline Rydog400f

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Re: Bent connecting rod?
« Reply #2 on: June 01, 2019, 09:58:57 PM »
Quote from: 754 on June 01, 2019, 09:53:22 PM
If it was running like that you will see a strange pattern on the piston.
 How did the dowel get bent ? It need changing.

I'm not sure how the dowel got bent because the cases came from a different bike. What would cause the rod to bend?
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Offline Tracksnblades1

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Re: Bent connecting rod?
« Reply #3 on: June 01, 2019, 10:13:05 PM »
Quote from: 754 on June 01, 2019, 09:53:22 PM
If it was running like that you will see a strange pattern on the piston.
 How did the dowel get bent ? It need changing.

Check the rod journal for damage also. Hopefully the rod bearing took all the misalignment wear..
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Offline Rydog400f

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Re: Bent connecting rod?
« Reply #4 on: June 01, 2019, 10:15:15 PM »
Quote from: Tracksnblades1 on June 01, 2019, 10:13:05 PM
Quote from: 754 on June 01, 2019, 09:53:22 PM
If it was running like that you will see a strange pattern on the piston.
 How did the dowel get bent ? It need changing.

Check the rod journal for damage also. Hopefully the rod bearing took all the misalignment wear..


Looks like I'll have to find a new rod and plastigauge it.
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Offline 754

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Re: Bent connecting rod?
« Reply #5 on: June 01, 2019, 11:31:31 PM »
Matched set of cases I hope..
 It may have got dropped or hit while apart.
 Rods are cheap, but can be straightened.
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Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline bryanj

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Re: Bent connecting rod?
« Reply #6 on: June 02, 2019, 01:19:25 AM »
If that was in a running engine the piston must have diagonal wear marks.
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Offline PeWe

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Re: Bent connecting rod?
« Reply #7 on: June 02, 2019, 01:23:26 AM »
Rod bent after case opened or at least cyl off? Dropped from the bench or when trying to lift it manually finding out it was heavier than expected...
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Rydog400f

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Re: Bent connecting rod?
« Reply #8 on: June 02, 2019, 08:58:42 AM »
Quote from: 754 on June 01, 2019, 11:31:31 PM
Matched set of cases I hope..
 It may have got dropped or hit while apart.
 Rods are cheap, but can be straightened.

I got a new matching set after I butchered the original block trying to remove the studs.
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Offline Rydog400f

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Re: Bent connecting rod?
« Reply #9 on: June 02, 2019, 08:59:48 AM »
Quote from: bryanj on June 02, 2019, 01:19:25 AM
If that was in a running engine the piston must have diagonal wear marks.

They all look uniform so maybe it happened when the PO took this apart to rebuild it.
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Offline Rydog400f

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Re: Bent connecting rod?
« Reply #10 on: June 02, 2019, 03:45:58 PM »
So the rod that needs replacing has an f,3, and S76 stamped on it. Can anyone tell me what stamps need to be the same for the new rod? Thanks.
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Offline PeWe

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Re: Bent connecting rod?
« Reply #11 on: June 02, 2019, 09:37:42 PM »
Something about weight marks. All need to have same weight. see page 51
http://www.honda4fun.com/pdf_man_officina/pdf_manuali/CB750_K-F/CB750_2.pdf
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline disco

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Re: Bent connecting rod?
« Reply #12 on: June 03, 2019, 06:41:56 AM »
I use 15mm diameter ground ‘silver steel’ rod to check alignment of conrods. If the silver steel won’t pass thru all 4 rods I know that something is bent.
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1976 CB750 K6 Sapphire Blue
1972 CB750 K2 836 Orange Sunrise
1972 CB750 K2 Candy Red
1972 CB750 K2 Candy Gold'

Offline Rydog400f

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Re: Bent connecting rod?
« Reply #13 on: June 03, 2019, 08:04:05 AM »
Quote from: PeWe on June 02, 2019, 09:37:42 PM
Something about weight marks. All need to have same weight. see page 51
http://www.honda4fun.com/pdf_man_officina/pdf_manuali/CB750_K-F/CB750_2.pdf

Figured out the mark's. Letters are for weight, numbers are for bearings. Got another F rod on ebay.
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