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Author Topic: 1975 CB400F Engine knocking  (Read 155 times)

Offline bonbons

  • Posts: 13
1975 CB400F Engine knocking
« on: Oct 04, 2018, 23:15:50 »
Iím currently in the process of tearing down my Ď75 Honda CB400F engine (splitting the case) for the second time to investigate a knocking sound that has been difficult to figure out where itís coming from. It sounds like metal hitting metal as the loudness and pitch of it would be indicative of that...
https://drive.google.com/open?id=1E5MqHdtQRm4Sx8NzEriQ9lVHqyqeydYj
Dropping the oil pan, I find no evidence of metal shavings not shards of anything. No sign of odd wear on the clutch nor stator. Even took a peek underneath and find that everything looks clean

On the previous engine tear down I uncovered this shearing on the exhaust side of the upper half on the engine

Was this once where the bottom part of cam chain guide seated? Did the cam chain grind it off?

Scratching my head hard on this one (and i bet itís gonna be something really dumb).


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« Last Edit: Oct 14, 2018, 20:37:23 by bonbons »

Offline trek97

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Re: 1975 CB400F Engine knocking
« Reply #1 on: Oct 14, 2018, 17:27:14 »
Could be the cam chain.

Im starting my 75 400f teardown/ rebuild this week.

Talking w Kanticoy yesterday he told me the 400f is notorious for sticking cam chain adjuster.

So I went ahead and ordered the improved aluminum adjuster, new chain, slipper and tensioner from David Silvers.

Hoping it arrives before next weekend.

Please keep us posted on your build w lots of good pics and any insights you can offer.

Offline bonbons

  • Posts: 13
Re: 1975 CB400F Engine knocking
« Reply #2 on: Oct 14, 2018, 17:37:15 »
With a micrometer I measured the journals, crankshaft mains and big end rods:

Main Bearing Journals
1 - 31.9777mm
2 - 31.9862mm
3 - 31.9862mm
4 - 31.9862mm
5 - 31.9887mm

Big End Rod Journals
1 - 31.9887mm
2 - 31.9887mm
3 - 31.9887mm
4 - 31.7830mm (up/down), 31.9887mm (sides)

The bearings spun on #4 and thereís noticeable slop there too, some 0.2057mm worth.



Iím convinced that this was the source of the knock. Crankshaft replacement is in order. Ended up ordering a whole crankshaft and rods set via eBay that was pulled from a Ď72 CB350F. Sources say that the 350F and 400F of that era pretty much shared the same engine components. Fingers crossed this will fit and work just fine.



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Offline trek97

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Re: 1975 CB400F Engine knocking
« Reply #3 on: Oct 14, 2018, 18:00:08 »
Pics of the replacement case I picked up...



Been chewed on some.  I dont know how deep it was new but its about 5/8 inch now.



I will have a second example to check by next weekend.

Offline bonbons

  • Posts: 13
Re: 1975 CB400F Engine knocking
« Reply #4 on: Oct 14, 2018, 18:02:33 »
Ah! Thanks for those pics. I guess then itís common that piece is worn down via can chain. I doubt that would be any source of knocking, however. It would just get chewed down as a result.


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Offline trek97

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Re: 1975 CB400F Engine knocking
« Reply #5 on: Oct 14, 2018, 18:09:00 »
Stinks about your bearings and crankshaft though.

You can get new bearings from davidsilversspares.com

Offline trek97

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Re: 1975 CB400F Engine knocking
« Reply #6 on: Oct 14, 2018, 18:14:32 »
I picked up plastigauge at autozone to check bearings one they go into the new case.

« Last Edit: Oct 14, 2018, 18:16:42 by trek97 »

Offline bonbons

  • Posts: 13
1975 CB400F Engine knocking
« Reply #7 on: Oct 14, 2018, 20:48:48 »
Good go on the green Plastigauge. Iíve never used the red version...

Yeah, seems that my #4 rod journal had gone oval and spun the bearing, leaving wear marks both on the inside and outside. I wonder if the oil passage had been blocked or something. Strange.

Iím awaiting parts from DSS - I wish they had a better package tracking system. Iíll be ordering some crank and rod bearings from them for sure


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« Last Edit: Oct 14, 2018, 23:04:58 by bonbons »

Offline trek97

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Re: 1975 CB400F Engine knocking
« Reply #8 on: Oct 14, 2018, 22:52:34 »
Good go on the green Plastigauge. Iíve never used the red version...

Yeah, seems that my #4 rod journal had gone oval and spun the bearing both on the inside and outside. I wonder if the oil passage had been blocked or something. Strange.

Iím awaiting parts from DSS - I wish they had a better package tracking system. Iíll be ordering some crank and rod bearings from them for sure

The red is just a bit thicker diameter for wider measures.

Yep I agree, they should offer better tracking.  Not a fan of the $20 flat rate shipping either.  Makes them out of the question for small orders.