help with identifying my mercury

[todd281]

Ok here is the lowdown , I have a mercury 3 cylinder motor that is either a 65, or a 70 horse . the way I got it, someone had already worked on it, so the cowling says 650, the lower end says 70 HP . I checked and the 65hp powerhead and the 70 could be interchanged. it has low compression on one cylinder , I took it apart and found that the previous owner has replaced one of the pistons, however the top two cylinders,that have good compression,have pistons with 3 rings each. the bottom cylinder with the poor compression has only 2 rings. so I am assuming that the lower was replaced with the wrong piston. Now because I don't have the proper model/ serial #s I am having a time finding parts. does anyone Know the standard bore sizes for these engines ? my pistons measure 2.880 and the bore is 2.885. more information;this is the motor with the reeds on the crank , and two carbs.

 

[merc850]

Old motors have the ser. Nr. here (3cyl too)

 

[Chinewalker]

The 2-ring pistons are newer style, the 3-rings are older style. Serial number will be in tag as shown above, as well as stamped (NOT CAST) in the block, usually up high, near the flywheel.

 

[jimmbo]

The 2 ring piston is actually an updated part. There are slight differences between the 65 And 70. The power heads will physically interchange. Since you have it apart, look for part #s on the pistons, carbs, and engine block. Armed with these a dealer may be able to determine what you have. Of course you may have a real college of parts from several motors

 

[racerone]

I do not believe a 3 cylinder 650 powerhead can be interchanged wiyth a 3 cylinder 700 / 70 hp powerhead.----They look the same, but are not the same.

 

[todd281]

I have a serial # but it is on the handle on the lower half. when you unbolt the powerhead, it is separated from it , so the powerhead could have been changed , and the serial # would still say its a 70 horse. I have looked for parts #s on the block but all I get are casting #s no good for identification . it seems odd that the piston with the two rings is also the one with the low compression. the cylinders all look good no scoring but the readings are 130lbs on the top and middle, 70lbs on the bottom... thanks for the input so far ,

 

[todd281]

I will go and look again for some stamped #s . I know they are sometimes on a freeze out plug but this engine does not seem to have any plugs . I will look up high as instructed though thanks

 

[todd281]

I just went and looked could not find any stamped #s just the cast ones

 

[racerone]

Is there a belt driven distributor , yes or no ?---Standard bore is 2.875.------It appears to me , from far away , that the motor may have been rebuilt at one time with 0.010" pistons.

 

[jimmbo]

I do not believe a 3 cylinder 650 powerhead can be interchanged wiyth a 3 cylinder 700 / 70 hp powerhead.----They look the same, but are not the same.

If they don't I stand corrected.

 

[racerone]

You would have to look at the part number for the gasket on the two models.----The gasket between the block and the exhaust housing.

 

[todd281]

there is no distributor , thunderbolt ignition .

 

[jimmbo]

kind of like the difference in the 115s in 72 and 73?

 

[todd281]

I found a website from Wiseco pistons that says the standard bore is 2.875 on the 65 hp made from 1973-76 and the 70 hp from1977- 83 . so I`m guessing that they just changed the carbs or the jetting . I am going to go verify my bore readings and then try to address why the compression is low on that cylinder with the 2 ring piston.

 

[racerone]

For a number of years MERCURY used a standard bore of 2.875 " on 2 cylinder , 3 cylinder , 4 cylinder and 6 cylinder engines with the same pistons too.-------If there is no distributor it is likely the 70 hp model with a 65 cowling.----Post pictures of any decals that say 70 hp.

 

[todd281]

well now I need a part # . I started to reassemble, and broke a ring on one of the pistons that has 3 rings . I did some research and I am getting conflicting data . the part # I got for the rings is 39-78816a12 or Sierra 18-3902. Some websites say that's a 3 ring set, some say its 2 rings . Marine Engine.com says its 3 on their site but its for the 3.375 diameter piston . NAPA says its a 2 ring set . Does any know the correct part # for the 3 ring set , standard size, 2.875 bore ,

 

[racerone]

I believe that part # gets you 12 piston rings.------If you could walk over I would give you a good ring.

 

[todd281]

geeze,, that would be great . I`m about ready to give up on this thing and but it up for sale on e-bay .

 

[merc850]

Buy 1 10 or 1,000 from here if standard size (measure piston diameter) https://store.oldmercs.com/category_s/1852.htm