1975 CB400F Engine knocking

bonbons

New Member
I’m currently in the process of tearing down my ‘75 Honda CB400F engine (splitting the case) for the second time to investigate a knocking sound that has been difficult to figure out where it’s 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 it’s gonna be something really dumb).


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trek97

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Re: 1975 CB400F Engine knocking

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.
 

bonbons

New Member
Re: 1975 CB400F Engine knocking

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 there’s noticeable slop there too, some 0.2057mm worth.



I’m 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 ‘72 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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trek97

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Re: 1975 CB400F Engine knocking

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.
 

bonbons

New Member
Re: 1975 CB400F Engine knocking

Ah! Thanks for those pics. I guess then it’s 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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trek97

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Re: 1975 CB400F Engine knocking

Stinks about your bearings and crankshaft though.

You can get new bearings from davidsilversspares.com
 

trek97

Active Member
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Re: 1975 CB400F Engine knocking

I picked up plastigauge at autozone to check bearings one they go into the new case.
 

bonbons

New Member
Good go on the green Plastigauge. I’ve 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.

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


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trek97

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bonbons said:
Good go on the green Plastigauge. I’ve 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.

I’m awaiting parts from DSS - I wish they had a better package tracking system. I’ll 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.
 

bonbons

New Member

Sorry I’ve not chimed in. I’ve been trying to fine tune the bike now after replacing the entire crankshaft assembly including the connector rods and all the bearings. So far so smooth on that aspect and most importantly, no more knocking!

New issue has arisen and that’s with 1st gear not engaging right. It clunks dangerously enough for me not wanting me to even try to engage it so I’ve been having to take off while I’m 2nd gear which isn’t the funnest. Gonna give this closer inspection thru the week, starting with a clutch adjustment (I doubt this is the issue) and then behind the clutch to check out that part of the shifting mechanism; before I have to go splitting the case again. I think I can split the bottom half of the lower case, right? I dread having to peel off the top cover head and cylinders to get to the transmission below...

Nevertheless, engine knock BE GONE!


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trek97

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Holy Smokes Batman. You work fast!

yeah seems the primary shaft, starter clutch and chain. Would be the biggest stuff w pulling the bottom case. Right?

You've done it more than I have. ;D
 

bonbons

New Member
Thanks! Yeah, I was pretty fast on this one but as they say, haste makes waste... though I did check things over as I do. Newish cam chain, newish clutch discs, brand spankin’ new primary chain too. I’m thinking it could be that one of my shifting forks is bent :-/ We shall soon see.

Meanwhile, I am riding on gears 2 thru 5 trouble free. Bike is smooth as buttah on those gears and is real fun. I hope to get this running at optimum again soon


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bonbons

New Member
More details on where I’m at with this build and how it’s come along since inception

IDK Numero Dos - 1975 Honda CB400F
https://r.tapatalk.com/shareLink?share_fid=13548&share_tid=159591&url=http%3A%2F%2Fforums%2Esohc4%2Enet%2Findex%2Ephp%3Ftopic%3D159591&share_type=t


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trek97

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bonbons said:
More details on where I’m at with this build and how it’s come along since inception

IDK Numero Dos - 1975 Honda CB400F
https://r.tapatalk.com/shareLink?share_fid=13548&share_tid=159591&url=http%3A%2F%2Fforums%2Esohc4%2Enet%2Findex%2Ephp%3Ftopic%3D159591&share_type=t
Aaaahh Thank you for that. I went through the whole thread (hit and miss) reading your posts that caught my eye.

That broken tensioner bolt. Ugh.

I was gonna suggest a change in oil. But I also run the rotella in all my bikes. Interested to see what you find w the shift forks.

You've done a bunch of good work to the machine. Pods can be a pain in the ass to tune. lol.

I know your shooting for the "LOOK" but Im keeping things stock. Its just easier and reliable for me. I aint getting any younger here.

Please Keep the updates coming our way.
 

bonbons

New Member
The picture may be a bit misleading but yes, the exhaust does ride low. No off roadin’ with this bike and I’m careful over speed bumps and dips.

The exhaust is handmade from Outex (Japan). I like it a lot.


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trek97

Active Member
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Long time no see bro. Hows things going?

I got busy yesterday and actually got a short ride in.

Check it out when you find the time...

http://www.dotheton.com/forum/index.php?topic=75640.msg912689#msg912689
 

bonbons

New Member
trek97 said:
Long time no see bro. Hows things going?

I got busy yesterday and actually got a short ride in.

Check it out when you find the time...

http://www.dotheton.com/forum/index.php?topic=75640.msg912689#msg912689
Awesome! Congrats! Though, at the least I'd say put a front brake on that thing!
Get these bikes out on the road!

Things with my 400F are meh; things can always improve. Still getting some rides in. Because of the air pods and aftermarket exhaust, I had to upside my jets (88 Mains; 75 is stock). I recently discovered too that it's important not to mix/match manufacturers of these carb parts. Since I had an aftermarket main jet, I kept its constituent parts stock (i.e. needles and emulsion tubes were Keihin which didn't seems to fit well with the aftermarket mains). This time I tried swapping out those parts to try to keep consistent with the aftermarket mains. It was noted in some other posts (TwoTired on the sohc4 forums) around that the needle taper and point/mating profiles are important to consider when messing with jetting. So I swapped out those carb parts and it is definitely a lot more steady and doing carb sync is seemingly a lot less laborious. Will get some more ride time in to see for certain.
Meanwhile, I'm anxious to drop my oil pan as 1st gear is still a big unruly. The way that gear behaves makes me thing that those gears are somehow out of round in the way that the bike feels like it oscillates at 1/4 to 3/8 revs. Strange.

Ride on.
 

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