What's the purpose of this hole?

[Don Hansen]

The photos show the reed valve assembly, the gasket, and the intake side of the crankcase of my 1974 4 hp. You see the hole in the center of the reed plate and the gasket. It leads to the pocket marked with an X on the crankcase half. That pocket leads nowhere. Anyone have any idea of what the hole is for?

DLH

 

[boobie]

Re: What's the purpose of this hole?

It's part of the internal oil recirculation system in the mtr.

 

[Don Hansen]

Re: What's the purpose of this hole?

Thanks, I think I understand how it works. It's interesting how simple the motor is in principle and the little engineering tweaks that make it work.

DLH

 

[seahorse5]

Re: What's the purpose of this hole?

The intake manifold vacuum pulls the fuel-oil mix from the bottom of the lower crankshaft bearing area, up though the recirculation hose to lubricate the upper crank bearing, then any fuel-oil buildup is drawn into the intake to be burned.

 

[Don Hansen]

Re: What's the purpose of this hole?

The intake manifold vacuum pulls the fuel-oil mix from the bottom of the lower crankshaft bearing area, up though the recirculation hose to lubricate the upper crank bearing, then any fuel-oil buildup is drawn into the intake to be burned.

There is no hose or any other path for a liquid to be transfered from the lower crankcase area to the upper main bearing area. I looked closely at the upper main bearing area, it's a needle bearing, and I can't find any way for it to be lubricated.

I don't know the history of my 1974 power head, found it on Ebay, but it was advertised with low hours. When I got it I was glad to see it had never even had the head off. When I looked in through a spark plug hole was I surprised to see the cross hatch still on the cylinder walls. I thought I had lucked out until I noticed it had a bad lower rod bearing. Hoping it would hold together for my Canadian trip I decided to use it anyway. Lasted about an hour before the bad rod cap screw came through the crank case.

Anyway, both my 1970 and my 1974 power heads are exactly the same. No way to feed liquid back to the upper main bearing and my 1970 has many, many hours on it. Something else, the 1974 has grease in the needle bearing and the 1970 does not. I'm thinking that the grease is to hold the needles in place during assembly and it was washed out long ago in the 1970. How I don't know.

DLH

 

[Don Hansen]

Re: What's the purpose of this hole?

I can see how the center and lower mains are oiled. Picture the 2 cylinders stacked one on top of the other as they are when the motor is on a boat. The gas/oil mixture will collect at the bottom wall of each cylinder and as the piston comes down, the rings will scrape the mixture into a machined pocket that leads to those mains. Upper to the center main and lower to the bottom main.

Now for the upper main. Seems to get oiled by what ever gets splashed up to it. At one time the was a vacuum operated method of oiling the upper main. And one can only guess as to why half of it still exsists. Looking at the picture, the blue box shows where the vacuum enters the channel through holes in the gasket and reed plate. At one time there would be a hole drilled from the yellow box, the direction of the yellow line, until it reached the bore that holds the upper main needle bearing. That vacuum, plus the pressure in the crankcase, would draw/push the gas/oil mixture from the crank case into the bearing. Both my 1970 and my 1974 power head do not have that drilled hole. There is a similar system for the lower main.

As for what happens to any gas/oil mixture that collects at the bottom of the crankcase the only thing I can think of is it leaks out past the lower main since there is no seal on the outside of that bearing unlike the upper main.

Don Hansen

 

[phillnjack2]

Re: What's the purpose of this hole?

From what im seeing there is no point of that hole being there at all.
if there is no other holes in the plate and no holes for any fluid to leak into the crankcase then all it can be is that the yellow plate if used on a larger or different model engine that does have the hole through to crankcase.
many engines have parts that are very similar to others, but often blanked off.
so unless you can find a pin hole in the casing its just pointless to your model from what i can see.


phill