Logo
FULL SITE MENU

1971 Cylinder head swap 18hp head to 25hp motor?

C

Cascade Classic Outboards

Member
So, I realize they are not the same exactly but can anyone confirm whether or not using the 18 head on the 25 will cause performance problems. I am not trying to turn the 25 into an 18. I just happen to have a head for an 18 and the 25 head plug hole is stripped and is not holding the heli-coil.

Thanks in advance.
 
Try the local parts houses for another type of insert when the heli-coil won't work. I know they make them.
 
Last edited:
I would strongly discourage using the 18 head on a 25. It WILL have an effect on compression. The 22 cubic inch engine was produced in three different horsepowers, 18, 20, and 25hp. In doing so, they made small differences in the heads AND pistons to arrive at their goal. If the head does not belong with the pistons installed, compression will be different, either higher or lower. I don't know offhand which would be the result in this particular combination. One might think that higher would be good (more power), but it might also cause pre-ignition or detonation which would destroy the pistons. Lower compression would cause lack of power.
 
I have no problem removing the head and putting a new coil in. I will stick with the Heli-Coil brand, but is there a specific type that I should be looking for for this type of head? Does anyone know if they make one that can be installed more securely with the head off?
 
Cascade Classic Outboards said:
I have no problem removing the head and putting a new coil in. I will stick with the Heli-Coil brand, but is there a specific type that I should be looking for for this type of head? Does anyone know if they make one that can be installed more securely with the head off?
Click to expand...

In your original post, you said the threads are stripped and will not hold a Heli-Coil. If that is true, the game is over as far as the Heli-Coil goes. You will have to find some other sort of insert or replace the head. I say let it go and replace the head with a correct one.
 
Back
Top