Topic: CB400F Engine Rebuild

[Alberta400f]

Hello everyone,

First a bit of intro, skip to the photos if you don't care.

I'm pretty new to this sub but I've gained some valuable information following other peoples builds and especially Nurse Julie's engine rebuild on the UK forums.

The bike I have is a 1975 CB400F that I bought as a first bike off of a friend for cheap. I planned on upgrading to a modern bike but I've always had the dream of restoring an old piece of machinery (I have a 1981 MK1 Scirocco sitting in a yard as well which I've had plans for, for years).

At first I thought the bike was in pretty good condition despite being neglected, however it has its own issues. I've been stranded on the side of the road in various places several times when the condensers have given out and dealt with stalling issues when hot at low RPMs. I brought it into a shop to see if they can't fix some of these problems since I didn't have time. This is when it started to become apparent that this bike wasn't quite as untouched as I had thought.

First it was obvious that the clutch cable was not original and had been routed rather incorrectly (this should have been some foreshadowing) but most worryingly we discovered there was an issue with the forks. The bottom fender stay was bent rather nicely but the shop owner pointed out that he was pretty sure the stays were straight on original 400s so I opted to have the shop check the forks. As it turn out the forks were very bent and not wanting to ride a bike with bent forks I got them straightened and rebuilt.

Those issues aside, obviously the bike has had "work" done but of the kind that made it worse than before. Additionally it burns a lot of oil and smokes from somewhere near the exhaust collector when its hot. Put all that together and I decided that if I was going to keep this bike I was going to tear it down and find everything I need to fix, my focus is on getting a solid rebuilt engine and a nice frame to put it all back in on. Personally, the other details like shinny new fenders and un-ripped seats are less important and I can fix those later down the road (and when the money's there).

So now for some actual progress and photos.

First thing that I found was this bolt that was conveniently slotted in to hide the fact it was the wrong one. Both sides were like this and I later discovered the head nuts? where not correctly reinstalled by a previous owner.



This engine is grimy and it only gets worse further down



Everything in this looks a little neglected, I think a gasket sealant was used and it has started to peel and I found little bits of it everywhere. Here you can also the special head nut in the wrong place.



There is already signs of damage and potentially a reason this engine was "rebuilt" at one point



My guess is the timing chain snapped I don't know how common that would be but how else would that score mark in the cover appear.

Also, in my opinion the rocker pad in the picture isn't looking to hot but this is my first experience with an engine rebuild maybe someone can comment.

This is the worst damage I found, besides these pads also looking sad. Something must have been flying around in the head at one point or someone was a little ham-fisted with a power tool



There is quite a bit of carbon build up as well and a lot of corrosion. I still haven't been able to get the dowels out of the head, they are strongly rusted in place.




The pistons don't seem to bad to me (compared to some I've seen in this forum) but there is a lot of corrosion around the studs so I plan to replace all of them.




At the same time I started disassembling the carbs. I had these rebuilt last summer so the plan is to replace the gaskets on the top covers and floats and get everything ultrasonically cleaned and vapour blasted




Now we come to my biggest concern, some thoughts on this would be great. The con rods are very obviously rusty, a bit curious as to how that might happen.




I'm not sure what to do about this, to me I should get new ones but new new are not available so I'd need to source nice used ones but I don't know what I need to look for, or do I get a whole crank shaft/ rod combo?

There is also more rust in the generator but this seems manageable
 

[Alberta400f]

Got around to working on this again the goal was to have the case torn down. Still deciding if I will get a shop to check the engine out or if I'll do it on my own. The engines pretty grimy so I might give it a good clean before I bring it anywhere, I'd feel a little bad about that.





The oil in this engine is black as night. I had a new oil filter put in at a local shop and I have a hard time believing the oil has gotten this dirty that fast, maybe they didn't put any new oil in it.



There is a lot of sediment at the bottom which has me concerned for the condition of the gears and oil pump if this has been circulating sand for a while.





I found little things like this everywhere, I'm fairly certain its whatever was used to seal the cases.

[jakec]

Keep the shells in order. Regarding the oil sump it looks normal to me. Simply doing an oil change won’t get rid of that stuff. The point is the sump is down low so all the bad stuff gets trapped down there. Since you split the cases you might as well rebuild the engine yourself. It wactually was very easy to put together, the hardest part for me was the prep work needed to get the cases ready.

[Alberta400f]

So its been a while. I have been working on it just haven't gotten around to posting an update.

I removed all the exhaust studs, only snapped one off  . I plan to replace them all with new since they were well corroded.

Disassembled the oil pump and checked the tolerances, I'd like to replace the gaskets once I find out their sizes, I haven't seen any kits. Going to throw the parts in the ultrasonic cleaner I got before I put it back together as well


Got all the engine studs out, not without great difficulty, luckily I'm planning to replace them all so I didn't need to worry about saving them. Most of them were very rusty.

Getting the studs out also help remove the old gasket. I heard permatex gasket remover works well but I could not find any. I tried, kerosene, brake cleaner, engine de-greaser and some Goo Gone. In the end it was a lot of elbow grease and carefully using a razor to cut the gasket away. I tried using a plastic scrapper but that was hopeless.

I also got some plastigauge to check the main journal clearances. I did it once with the con rods still attached and then again without because I found it easier to get the crank in and out of the case.



To me the bearing clearances look fine. Spec is 0.018 - 0.048mm and the service limit is 0.08 mm. The plastigague seems to suggest at least 0.038 mm clearance. The mechanic I brought all the cases to, to get vapour blasted felt like I need new ones.

What concerns me is the con rod big end side clearance should be between 0.02 - 0.07mm with service limit of 0.15mm. I unfortunaly don't have a photo but a 0.15mm feeler gauge fit no problem. Has anyone check this tolerance before? Should I be concerned about this? Fixing this seems....impossible... without replacing the whole assembly

I also plasti-gauged the con rod big ends. The specs are the same as the main journals (0.018 - 0.048mm and the service limit is 0.08 mm). Unfortunately here it seems like it is out of spec. The result was more or less the same for all of the con rods so I think I will for sure be replacing these although there is some room to the service limit. May as well do these while I this far in and if I can find them.


For the valves I splurged and bought a valve spring compressor and I'm glad I did. Removing the valves was very easy and now I have one for future projects  .



The valves were very heavily carbon-ed, again the mechanic took one look and said I'm going to need all new valves so I'll see about ordering new ones. I've found some on david silver spares, any thoughts on genuine versus aftermarket options?




Found some more evidence that someone has been in this engine before, looks like instead of geeting the right gaskets they decided to fill these grooves with something blue. Luckily it was easy to remove.


While scrubbing the cases to save some labour at the vapour blasters I came across this hole. Seems like the drilling for one of the case bolts went a little too deep, very curious, wondering if anyone else has seen this on their cases?


Currently the cases, covers, barrels, head and carb bodies are at the mechanic waiting to be vapour blasted. I took some before photos, looking forward to the comparison. Now I'm looking where to source parts and all the parts I will need.

I will get an overbore kit (.25mm, I'd like to leave room for future fixes), does anyone have experience with these. I found cruzinimage, david silver spares and cmnsl all have some available for widely different prices, is there a difference in quality of any of these?

[jakec]

That hole is normal, at least my engine has it.

Be careful, my vapor blasting came out to $500+ and the top end work $700+ haha!   

[Alberta400f]

That hole is normal, at least my engine has it.

Be careful, my vapor blasting came out to $500+ and the top end work $700+ haha!   

Yeah based off the discussion I had with the mechanic the cost is going to be similar for me which hurts a little but I have a hard time stopping myself from trying to fix everything wrong haha . The way I see it I'd rather do it now than later

[Alberta400f]

So I went to take apart the rest of the cam cover,

There was some damage to the cover near by one rocker that I had seen early on. Looks like something tried a little nibbling.


However the damage was worse that I expected. Aside from the cosmetic damage it was extremely difficult to get the rocker shaft out unlike the other three which more or less slid out. After getting the shaft out you can see that whatever happened actually cracked the cover 


Maybe a wizard could fix this with some welding and serious machining but for me I guess I will need to source a new cover. I'm worried about reusing the shaft as well as it possibly has been damaged as well, however it may also have been replaced by the previous mechanic.