Topic: 72 cb500f engine rebuild

[Chrispy56]

I recently picked up a 72 cb500/f for a good deal. The previous owner started a café project out of it and gave up. I will continue this project as a café but plan to keep it very simple and mostly oem components. While discussing details of this bike the previous owner had stated that he had torn the motor down "just because"...? reassembled, painted it and put it back in the frame. I have noticed gaskets missing causing oil to leak and an assortment of parts in a box that didn't make it back in the motor.  Everything else on the bike is there. brakes need rebuilding and needs a seat. other than that it is in good mechanical shape. 
     So, with that said I would like to first tear down the motor for inspection. I understand that the 500 didn't have the greatest transmission setup, but am not sure of the specifics. what should I be looking for? where are the failure points? are there heavy-duty replacement parts available? any recommended parts suppliers?

Thanks for any input,
-Chris

[Chrispy56]

This is what I am starting with. I found it for $200, I thought it was a good deal and a fun project.

[Trad]

That thing looks pretty cool. Good luck with the build.

[bryanj]

The only specific failure item is the primary chain. selectors and gears can wear and some are not available.

A cheapish upgrade is a complete 550 motor which will drop in and bolt up you only need the R/H rear mounting plate as an extra from the 550 as the clutch cable stop is on it

[Chrispy56]

The only specific failure item is the primary chain. selectors and gears can wear and some are not available.

A cheapish upgrade is a complete 550 motor which will drop in and bolt up you only need the R/H rear mounting plate as an extra from the 550 as the clutch cable stop is on it

Yes, I understand that is probably the best way to go. And I have been searching for a complete 550 motor.

Are the external components compatible between the 500 and 550? ie starter, intake and carbs(same size), points, etc.

I found a motor with about 10,000miles on it, but needs a little head work and mounts(including R/H rear mounting plate) for around $500 shipped. Worth it? Will the 550's more reliable bottom end give me the peace of mind I am looking for? I will go this route for reliability reasons only.

[Mo]

$500 seems like a lot. You could buy an entire 550 project bike on craigslist.

[Chrispy56]

I would settle for a 550 bottom end but can't seem to find one.

[Chrispy56]

Finally found a 550 motor. I haven't seen it yet, but the guy says that it is "locked up".

Assuming that it is repairable, what is it worth?

And will the majority of my 500 internal component be compatible with the 550? ie crank, rods, etc.

Thanks

[Chrispy56]

Found a different 550 motor. Researching the serial number tells me its from a 77 f model. Its complete, turns freely, and came with the complete wire harness w/no cut wires. He also had the front section for the 4 into 1 exhaust including the finned clamps.   

[nhg2112]

I would get the 550 motor and build it from the best parts out of both. As mentioned earlier - the important bits of these engines are interchangeable. If the 550 turns and has good compression then pick the best carbs/pipes you've got and go forward from there. Keep in mind the carbs are unique to the intake manifolds so they need to stay together as a pair. The 500's, while cool and unique(ish), can suffer issues that really arent worth it (primary chain, clutch & transmission). If you end up having low compression, rings & a hone are an easy enough job - easily done in an afternoon if the engine's out of the bike.

I would also be very weary of a project that someone 'tore down and then half-rebuilt'. Best case you'll have to tear it back down and rebuild it making sure he didnt miss anything, worst case you blow up your engine after investing time & money to get it running. Once you've got a running 550 engine you can tear down the 500 engine and part it out - you could probably get 2-300 out of it that way, while learning about these awesome engines in the process.

Let me know if you end up parting out the 500 - I'll buy your transmission if the miles are low enough. I was stubborn enough not to do the 550 swap when I started my build 

[Chrispy56]

I would get the 550 motor and build it from the best parts out of both. As mentioned earlier - the important bits of these engines are interchangeable. If the 550 turns and has good compression then pick the best carbs/pipes you've got and go forward from there. Keep in mind the carbs are unique to the intake manifolds so they need to stay together as a pair. The 500's, while cool and unique(ish), can suffer issues that really arent worth it (primary chain, clutch & transmission). If you end up having low compression, rings & a hone are an easy enough job - easily done in an afternoon if the engine's out of the bike.

I would also be very weary of a project that someone 'tore down and then half-rebuilt'. Best case you'll have to tear it back down and rebuild it making sure he didnt miss anything, worst case you blow up your engine after investing time & money to get it running. Once you've got a running 550 engine you can tear down the 500 engine and part it out - you could probably get 2-300 out of it that way, while learning about these awesome engines in the process.

Let me know if you end up parting out the 500 - I'll buy your transmission if the miles are low enough. I was stubborn enough not to do the 550 swap when I started my build 

My 550 didn't come with carbs but my 500 did along with the manifold. But I can't find any numbers on the carbs so I can't confirm what they are. Where are the numbers located?

[nhg2112]

This area right here. Check both sides

[bryanj]

To put the 550 motor in you will need the R/H rear 550 engine mounting plate as it has the clutch cable mount on it

[Chrispy56]

To put the 550 motor in you will need the R/H rear 550 engine mounting plate as it has the clutch cable mount on it

Yes, I received a box of parts with the motor, and found that piece in there.

[Chrispy56]

I cant see any casting information on the carbs (other than Keihin Japan)

Also, two of the four towers where the main jets go look to be corroded. I have never worked with this type of carb, and was surprised to see that the main jets are held in place with orings the two corroded towers are barely holding them. Are these worth rebuilding.

size of main jets are 100, if that helps in identifying them.

[Chrispy56]

The part number stamp was very light, but I did finally find it...its 627B. 71-73 CB500 Four.

[bryanj]

The main jets should be held in with a leaf spring afair, the O ring just seals

[Schroeder]

This is really interesting to read. I actually bought this bike from Chris and am in the process of rebuilding it with that 550 motor. Really glad that it was torn down when I got it. Learning a ton by going through manuals and seeing what is there and what is missing. A bit aggravating not having seen it come apart but it was fairly complete when I got it. Just getting into the carb rebuild now.

Roller with new bearings and seals, tires, etc.:

Pistons cleaned and rung:

Valves lapped and reinstalled.

Lots of oily dirt to clean up!

[bryanj]

A favourite oil leak is from the oil pump cover and you need a 46 x 2 mm O ring to fix it (NOT a 500/550 part but if you can't get a generic one the Gold Wing uses that size) also if you fit any sort of O ring chain you will eat away the crankcases and that rubber blanking plug in front of the sprocket

[Schroeder]

Thanks Bryanj. Ya there is a bit of wear on that oil plug from the chain. It appears to still be containing oil but we will see. I do have it on my list to rebuild the oil pump and put new runner in it. That seems to be a source of oil under the left side cover.  Thanks for the input.