1975 Mercury 850 rebuild powerhead or not?

[winded2002]

Serial 4235590. I think it's a 1975. I have it apart and found what I thought I'd find. The water pump impeller is shot. Top cylinder of inline 4 is not scored, I think. Rings are pretty worn on that cylinder and piston looks a bit ugly. The other 3 cyl. look good and compression was good.... My question is, since I have it apart, does it make sense to replace other moving parts...bearings, wrist pins..., even if they're working fine. Or do I leave well enough alone. I'm not an experienced rebuilder, I just want to replace anything that should be replaced since I have it all apart.

 

[sho305]

Re: 1975 Mercury 850 rebuild powerhead or not?

IMHO, I would just fix up that cylinder and check everything else closely. Unless you think it has lots of hours on it, up here in the snow belt they take a long time to wear out.

 

[rickdb1boat]

Re: 1975 Mercury 850 rebuild powerhead or not?

winded2002,<br />Just to verify, it is a 1975 Model. I would do as suggested and just do the one piston, also. <br /><br />Good merc. site/with parts

 

[mikebob]

Re: 1975 Mercury 850 rebuild powerhead or not?

I have a 73 85hp merc and the water pump went out on me and it scored 2 pistons 1 and4. I just replaced them and put new rings on the other 2 because they still had good comp. it runs really good just like new and they have the same comp as the new. just remember too double your oil mix for at least two tank fulls and only run at half throttle for a couple of hours and then just short periods of full throttle for another 8-10hrs. good luck