Powerhead swap between 68 18hp, and 75 25 hp

[Rwed]

I wanna swap PH on my 68 18hp, and found this online. are those 2 compatible? Also, what to look out for, to make sure that the donor ph is good enough? and, also, what other materials would i need to complete the swap? TIA

 

[jimmbo]

Which is your motor? The sad case on the left or the assembled one on the right?

compression test
obivious signs of crankcase leaks
new gaskets

 

[racerone]

The one on the RIGHT will not turn I believe.

 

[Rwed]

racerone:correct, the one on the right is the one i wanna make run, other one is on fb, i'm trying to buy the ph from

 

[Rwed]

jimmbo: so, are the ph same on both? which gaskets would i need to do the swap?

 

[jimmbo]

Gasket between powerhead and midsection. If they both use same gasket, they should swap. Carb gasket will also be needed

 

[hardwater fisherman]

Just so you know the ignition systems are different on these two motors....

 

[F_R]

Cylinder heads are also different. OK if you are swapping the complete powerhead, I suppose. But don't mix powerhead parts. An 18 is NOT a de-tuned 25

 

[Rwed]

About that ignition systems being different, what does that mean? as far as swapping ph?

FR: I am planning on a complete swap. i didn't want take ph apart if possible, that's why i'm entertaining the swap idea.

 

[racerone]

Makes no difference.-------BUT,-----You do have to evaluate the condition of the replacement.-----Be a shame to do all that work to find out it is a dud.

 

[Rwed]

Racerone: to make clear. I can use the flywheel points ignition on this ph, right? no need to upgrade ignition system?

I have videos of compression testing from the guy, it reads 120psi on both pistons.
HE is supposed to take it off this week, and I was told to have him check inside the
ph, from underneath for scouring, is that right?
Aslo, anything else to look out for?

 

[hardwater fisherman]

The lo-tension magneto on the 1975 model was not an upgrade in my opinion....I think the mag plates will be different and the flywheels will have different amounts of magnets.... But may be interchangeable.....

 

[Rwed]

So, can i put the mag plate on top of the 75 ph? and just run with my points ignition? like, theres the mounting bolt holes for the mag plate... are they spaced the same?

 

[F_R]

Unless there is some "gotcha", yes the mag can be swapped. You have to also swap the flywheel. Pretty sure the breaker cam also (gotcha)

 

[silver_bay]

I seem to recall that the 25HP exhaust housing (midsection/leg) includes a baffle for exhaust tuning, to create an expansion chamber to send a pressure wave to force fresh fuel/air mixture that followed the exhaust back into the cylinder.

 

[jimmbo]

I seem to recall that the 25HP exhaust housing (midsection/leg) includes a baffle for exhaust tuning, to create an expansion chamber to send a pressure wave to force fresh fuel/air mixture that followed the exhaust back into the cylinder.

No there was no differences in the exhaust between the 25 and the 18/20 of the era. The differences were; Pistons had the wrist pins located closer to the crown/top. That changed the effective port timing and reduced the compression ratio. A cylinder head with a smaller combustion chamber was then installed to restore compression. A bigger carb, larger intake manifold and modifications on the reeds and reed plate, a more finely balanced crankshaft was used on the 25 to tolerate the higher Rpm range of the 25.

many people have tried to stick the 25hp head on an 18/20, thinking they are converting it to a 25. What they succeeded in doing, was to sentence the engine to death. The result was too much compression and the engine detonated itself to death

 

[flyingscott]

I would leave it the way it is. For some reason the low tension magneto is not as big an issue on the 22 did block. The 9.9/15 were bad but I have owned several of the 74-76 22 did blocks. And the ignition worked pretty well on them.

 

[racerone]

That low tension magneto was installed on the FIRST year of 9.9 production.-------It was installed on a proven 22 cube design as well.--------Many of the early 9.9 issues were not caused by that magneto set-up.

 

[Rwed]

silver bay: what does that mean for the ph swap?

 

[Rwed]

silver bay: would that affect the swap?