Topic: 350/400 Hybrid

[wingman]

I recently split the cases on my 7.5k mile 350 for a rebuild.  When I pulled the rocker cover, I noticed some strange wear on a rocker and a fingernail feel-able scratch on the bearing surface for the cam in the head.  Otherwise the engine appears to be ok.  I am going to run the rocker box, cam and head over to a machine shop to have them look at the wear.

I was considering getting a top end from a 29k mile 400f (cylinders up) and putting it on my 350 cases.  I believe that the connecting rods are the same in the 350 and the 400, so I should be able to continue to use my crank and connecting rods (and according to my Clymer's manual - my wrist pins).  If I did this, I would not overbore and would stick with the stock 400 pistons and rings (assuming wear they have worn ok).  The oil pump is the same and I expect that the clutch could hold the additional possible 4-5 HP.

I won't have the 6th gear the 400f, but I think I would see a small (cause everything with these engines are small) bump in HP and torque. 

Has anyone done this swap or can comment on this?

[Triffecpa]

I've got one that I am assembling right now.  The cylinder and heads and pistons all fit with the exception that you need to bore the case to get the larger piston liners to fit into the crankcase.  I did mine somewhat crudely with a dremel after removing the upper cylinder studs from the crankcase.  (stuffed rags around the connecting rods and then used a vacuum to suck the shavings out)  It's amazing how much material that you have to take out to get the 400 cylinder to fit into the 350 crankcases.

the only reason that I did this is that the original 350 engine was frozen when I got it and the cylinder liners would have required a bore job to clean them up enough to make them usable (and I had an extra 400 cylinder and pistons)

I think that the 400 has a half millimeter larger valves (or some such unmeasurable difference) but the 350 cam is the same part number as the 400 one.

so, it's a somewhat bigger project that simply dropping the new top end onto the 350 cases.  I didn't have a lot of choices in my case though.

Tracy

[Triffecpa]

Latest update.  I first attempted to use the 350F head, which was a direct bolt on over the 400F cylinders.  Unfortunately the 350F head has a couple of bent valves (might explain why the bike was abandoned with only 5500 miles on the clock), so I cleaned up an extra 400F head and bolted it on.  It fits fine, but the front two center crankcase studs are too long.  There is a difference in the casting between the 350 and 400 heads.  I didn't have any extra 400F crankcase studs, so I made some spacers and dropped them over the studs (on top of the copper sealing washer) and then ran the head nut down.  We'll have to see later this week if it leaks or not.

Tracy

[camelman]

If you have extra parts laying around, then fitting a 400 head onto a 350 isn't a bad idea.  However, if you have to buy parts to complete the process, then I think you would be better off with an entire 400f engine swap into the 350.  Those engines can be had pretty cheaply these days.  Remember, you will also need a complete gasket set for the rebuild.  Not just the head and jugs.

Just my $0.02

Camelman

[fmctm1sw]

I'm getting closer and closer to trying this.  I pulle apart a 400F case a while back with a seized connecting rod.  I almost trashed it but them looked at the transmission and it looked good and the fische lists the same part number for the crankshaft.  So the game plan is to assemble the 400F cases with the 6 gear transmission and put a 350F crankshaft and top end on it.  Long way off but perhaps do-able..

[schwebel]

Isn't the combustion chamber larger in the 400f to accommodate the wider pistons? Also I know the valves are a little different,...larger. Wouldn't you run the risk of hitting the valves against the pistons and change the compression ratio running a 400f head? As said before, wouldn't it be a lot easier just dumping in a 400f lump. You would get the 6th gear, revised cylinder head and wider bore. I paid $200 for my running 400f engine. Only thing wrong was a leaky tach seal, I know my 350f is a lot happier now. I have however heard of some people running a 400f bottom end with a 350f top end with great results.

[wingman]

I was thinking about doing this after I saw my rocker box and the condition of some of my valves.  Trying to find parts for any of these smaller motors locally is hard.  I would love a 400 with a big bore kit - but right now, I am just going to fix up the 350.  I found some replacement valves on ebay and according to the machine shop, I should be in business.

[mlinder]

Isn't the combustion chamber larger in the 400f to accommodate the wider pistons? Also I know the valves are a little different,...larger. Wouldn't you run the risk of hitting the valves against the pistons and change the compression ratio running a 400f head? As said before, wouldn't it be a lot easier just dumping in a 400f lump. You would get the 6th gear, revised cylinder head and wider bore. I paid $200 for my running 400f engine. Only thing wrong was a leaky tach seal, I know my 350f is a lot happier now. I have however heard of some people running a 400f bottom end with a 350f top end with great results.

I'm pretty sure cam is the same, so valve to piston clearance should be more or less the same.
If going with 400 pistons and the 350 head, yeah, combustion chamber 'squish' area should be machined to out to diameter of the cylinder.
Yes, a complete swap to a 400 engine would prolly make more sense.

[carnivorous chicken]

Hi All,
   Great thread and particularly timely and relevant for me as... My CB350F engine appears to be taking a dump. Rather than a rebuild, I'm interested in swapping engines and throwing in a 400F engine (I miss my 400F, especially 6th gear!). My 350F is in such great shape cosmetically (and I actually prefer it's look -- tank seat and sidecovers) so I want to hold on to it. It's already running 400F exhaust and pegs. My (perhaps stupid) question is this, since I don't have a CB400F to make the comparison in person: Are the engine mounts identical, or is it going to involve welding new mounts to wedge the 400F engine into the 350F frame?
Thanks

[wingman]

Mounts are the same - should be a nice simple swap.

Just came to mind - you will probably need to rejet (but you probably knew that already).

[schwebel]

Since you have CB400f controls you have fixed the only issue with the swap. Which is the rear brake pedal clearance. The cases on the 400 are a little wider and make the brake pedal on the 350f harder to use.

You only have to change the slow jets. I just stole the ones from a CB500f (they are the same as the 400f's). Also turn out your airscrews 2 1/2 turns. All mounts and wiring is the exact same. It is plug and play. It took me about 2-3 hours to do this swap (with my brother helping). That included tuning up the engine as well.