Johnson 5.5hp

[Super Sportster]

I'm doing a powerhead swap, a CD20 is replacing a CD15. I've swapped out the head and intake manifold. Problem is the thermostat. I now have an exposed hole in the block. If I tap the hole for a pipe plug and plug it off will I still have cooling circulation like the CD15 would have?

 

[1946Zephyr]

Re: Johnson 5.5hp

It's best to use the same style powerhead that was used with the motor, originally. Those with thermostats have a different set up in the water passages, than those without thermostats. So, in your case, you want to stay with a powerhead that is 1956 to 1958. 1954 and 55 is a different style motor leg and may not work and 1959 and later is built with the thermostat and fiberglass shrouding, therefor making a lot of differences too.

 

[Super Sportster]

Re: Johnson 5.5hp

They are both parts engines, the CD 15 runs like crap, the CD 20 runs great. I'm not getting to deep into this project, it's just something to play with. So if no one knows with certainty whether or not this will work I guess I'll find out soon.

 

[F_R]

Re: Johnson 5.5hp

The thermostat-controlled blocks have two flow paths through it for the water. If you block one of them off, that part of the powerhead will overheat. Short story--if you are trying to use a non-thermostat cylinder head on a thermostat block, it won't work.

 

[Super Sportster]

Re: Johnson 5.5hp

I've looked it over a bit more and I see what you are saying. Looks like with the early head I no longer have the return to the exhaust port for discharge. I would need to link the the center vertical passage in the the head to the discharge port as the thermostat head does. That vertical passage has a plug in the top, so if I remove the plug and pipe it to the open hole I may have my problem solved. I'll do a little more investigation into this. Thanks for the replies! ****

 

[Super Sportster]

Re: Johnson 5.5hp

Upon further investigation the water inlet between the two blocks are the same. The discharge is different because of the thermostat so it actually is an easy fix. I drilled the passage that wasn't drilled in the late block as it is in the early block. So now I just need to plug the hole in the side of the late block like the early block is and plug the the thermostat hole. Water will flow now from the top of the block on the port side and discharge at the top of the block on the stbd side, just as the early block does.

 

[Super Sportster]

Good to go

Good to go

I've installed a 7/16" soft plug into the block where I've drill out the passage then tapped the open thermostat and threaded in a bolt with a copper sealing washer. I'll fire it off soon and see what happens!

 

[Super Sportster]

Missed one

Missed one

Missed a hole and plugged it with a brass rod. It seems to work okay, water is circulating! http://www.youtube.com/watch?v=FSu0GhyrWls Now on to the carb mods.

 

[F_R]

Re: Missed one

Re: Missed one

Good Grief!! And good luck

 

[14ftgrumman]

Re: Johnson 5.5hp

Wouldn't it have been easier to use the cd-20 head?

 

[Super Sportster]

Re: Johnson 5.5hp

You bet it would have been much easier to use the CD20 head! Now comes the "but", since it's all going into the CD15 chassis I needed the head with those ears sticking off it for the hood. I didn't get the hood with the CD20 and the ears on both sides of intake manifold were busted off where the recoil bolts up. So now I've got something that runs great, everything works, not a lot of $$ in it, lots of time and a sense of accomplishment.

 

[Super Sportster]

Johnson 5.5hp finished

Johnson 5.5hp finished

Mechanically sound, cosmetically challenged!