CB Engine Swap. Can it be done.

C

cb500fourish

New Member
Hey guys. This is my first post, so go easy on me.

I currently have a 1971 CB500f with a locked engine. It's all original with title and 13k on the bike. I also have a 1981 CB650 Custom with a good engine and no title.

My question is, will the '81 650 engine setup fit in the '71 500f frame? If so, what mods will I need to do exhaust-wise? Has this been done?




Here is a pic of the bikes, I'll take more when it's clear outside. '81 650 Custom (left) '71 500f (right)
 

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Re: CB Engine Swap. Can it be done.

Will it drop in? No. Is it possible? That depends on you...
 
I guess the question of "would that swap be worth it"? As far as welding in new brackets and such for mounting, I'm good with that.
 
There would be quite a bit of work involved in that swap. I would really suggest repairing or direct replacement.
 
Might be easier to transplant the steering stem of the bike that has a title onto the bike that runs.
 
Engine drops right in, no issues. Cyl is 5mm taller on 650 so using 500 exh might be a tight fit and poor exh seal as 650 head has larger exhaust ports. Spacing is the same though.
As you have an 81 650 it probably has CV carbs which may mean you need to use the complete throttle assembly and linkage from from the 650.
The last issue would be gas petcock on the 500. I think it will be fine and clear the 650 head with room for the fuel lines to run without kinking, otherwise use a later (77/78) model CB550 tank which will have no issues for clearance.
You will need to swap over the coils and ignitor coil triggers from the 650 as it is electronic ignition
Easyiest way to put in 650 engine is take off top cover from cyl head, take off oil filter, remove oil pan. Bolt it in using stock motor mounts and spacers.
 
Kemp said:
Engine drops right in, no issues. Cyl is 5mm taller on 650 so using 500 exh might be a tight fit and poor exh seal as 650 head has larger exhaust ports. Spacing is the same though.
As you have an 81 650 it probably has CV carbs which may mean you need to use the complete throttle assembly and linkage from from the 650.
The last issue would be gas petcock on the 500. I think it will be fine and clear the 650 head with room for the fuel lines to run without kinking, otherwise use a later (77/78) model CB550 tank which will have no issues for clearance.
You will need to swap over the coils and ignitor coil triggers from the 650 as it is electronic ignition
Easyiest way to put in 650 engine is take off top cover from cyl head, take off oil filter, remove oil pan. Bolt it in using stock motor mounts and spacers.
Click to expand...

Listen to Kemp. His Honda is one of the nicest machines I've ever seen.
 
Is there more information for Honda CB's generally.
For example what engine typically fit in which frames?
Could someone rate them 1.impossible, 2.very hard, 3.tricky, 4.minimal modification, and 5. slots right in.

How in terms of twins, about CB350 into CB250N, CB450 etc etc
 
99% of the time you CAN'T just swap out motors. Basically, the chances of a motor dropping right into a different frame are VERY slim to none.

That said, if it doesn't bolt right up figure its going to be a massive undertaking to make it fit.
 
Get some wrenches and try it for yourself.
Or with some google you can find out that the sohc Honda has a great forum filled with info.
OR you could search and see how guys use 650 stuff to improve their 500/550 bikes.
 
Lewn said:
I1.impossible

No such thing.
Click to expand...

I would recommend taking the 500 engine out. Wether you're going to replace or repair, it's coming out.
Check the damage. if it can be fixed easy, use cam and carbs from 650 and go from there.
thats the easiest.

swapping engines will work mounts-wise but like said before, you've got electrics, gasoline, carbs and exhaust as well.
depends how bad you want it and how much time/money you have.

500 engines go for 200/300 around here (dunno about there), i bet it will cost more to do the swap completely.
another thing to keep in mind is that you're basicly destroying a perfectly fine 650... you could also sell the 650 and use the cash to fix the 500.... So many options, so many choices :)

but i'd say:

DO IT!

aaaaand take pictures along the way and post em here so anybody with the same problem/idea can learn from you!
 
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