Do the cylinders on a '71 115 have factory taper?

[FoolOfAFisherman]

I have a '71 1150 115 SN 03027266 that needs two cylinders bored .015 oversize. I miced the cylinders with a bore gauge and found them to have .003 variation from bottom to top. (2.872 - 2.875) Is this factory or is this just wear. My service manual talks about taper but not to specifically. When I bore these cylinders, do I need to keep the taper or can I just bore it 2.890 all the way down? I'm a machinist with all the necessary tools and knowledge to do the job. I know the pistons are tapered and too much slop in the cylinders can cause the piston to slap, so I think I know the answer. Just curious what some of the marine machinist's have to say? Any info is much appreciated. Thanks.

 

[merc850]

Re: Do the cylinders on a '71 115 have factory taper?

I have read thru 2 manuals and there is no mention of tapering, I think it's just wear; as the connecting rod changes sides at the top the piston moves from side to side. Mercury also used "cam turned" pistons that were out of round when cold, then became round at oper. temp. so if your pistons seem out of round this could explain it.

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

Yeah, I haven't read anything saying that there's supposed to be taper but I do have a diagram that shows how taper effects the endgap in the rings. I just thought that the wearing it is doing is odd. I'm getting 2.872-2.873 in every cylinder at the bottom measuring across at "wagon wheel" style. 2.875-2.876 at the top of every cylinder measuring across the same way. If the cylinder is supposed to be 2.875, how can you get smaller? Just seems odd. The piston and cylinder specs in the back of my service manual don't specify any tolerances to go by and it doesn't say anything about taper so I'm not going to put it in there. I'll just bore the 2 oversize right on 2.890 -.000 +.001.

 

[Faztbullet]

Re: Do the cylinders on a '71 115 have factory taper?

Is this factory or is this just wear.

This is wear and I aint figured out how to taper bore a cylinder as my bar (FW-II) has a fixed bit??? You want to bore it a tad smaller as the 2.890 is the finished hone diameter not the bore to spec.

Mercury also used "cam turned" pistons

Dont think this was avalaible in 71...

 

[TomSawyer]

Re: Do the cylinders on a '71 115 have factory taper?

My Seloc manual says that .003-.004 is allowable taper. (Measure the top then the bottom and subtract.) My manual covers 1965-1989 3 and 4 cylinder 2 strokes.

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

This is wear and I aint figured out how to taper bore a cylinder as my bar (FW-II) has a fixed bit??? You want to bore it a tad smaller as the 2.890 is the finished hone diameter not the bore to spec.

Dont think this was avalaible in 71...

You can't taper a bore with a boring bar. You need tapered endmills that interpolate down the cylinder. CNC only i'm sure. They have tapered reamers that go up that big but it's not used for this purpose. We do mud pumps for National Oil Well Varco that have tapered cylinders, so it can be done.

 

[CharlieB]

Re: Do the cylinders on a '71 115 have factory taper?

Don't run enough coolant to your boring bar, the cyl can heat and the bore will have a taper!

Don't recommend that.

Like Fazt said, don't forget to leave enough for honing to final finish.

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

Thanks for the info, will do. Do you guys have a marine handbook you go by or is most of the stuff knowledge that's been passed down (which btw is what I prefer..you can learn a lot of from others mistakes ). We have a Machinist handbook that covers anything and everything pertaining to metal cutting. Just curious about the marine side of machining.

 

[CharlieB]

Re: Do the cylinders on a '71 115 have factory taper?

Boring and hone are about the same on all engines, the major difference in a two stroke is treatment of the ports, they need a slight champher so as not to leave a sharp edge that could snag a ring. The old machinist I learned from taught me to 'break' the edge on everything I machined with a file. Small 'finger' files work well to clean the ports. Some may use a small stone. Both work, just depends on the hand that does the job.

Piston to cyl clearance and final hone finish depend on the factory specs for that model/yr and piston supplier, cast vs forged.

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

The Specs for this year and model show 2.864 for the skirt, 2.857 above the rings. Cylinder hone finish is 2.875. That's a good amount of room between the piston and the cylinder wall. I knew the piston would expand but that's a lot. Btw, about chamfering everything, that's a good shop practice. Those old guys do know their stuff. That very same thing was told to me as well. You'd be surprised how many inexperienced machinist's skip deburring. Most of it is laziness because it definitely takes finesse. But, you know as well as I, what will happen if you skip something that looks like a small deal. And that can go for many different things as I've recently learned rebuilding this motor. Take your time and do it right because you don't want to have to do it again.

 

[Faztbullet]

Re: Do the cylinders on a '71 115 have factory taper?

Specs for this year/model show 2.864 for the skirt, 2.857 above the rings

Those spec's are incorrect. The correct spec's are : O.D above ring land...2.863 and O.D at skirt..2.872

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

Those specs I quoted were from the piston and cylinder specifications of a Seloc repair manual. How come the numbers you gave were so much different than the numbers from the service manual I have? (not that I'm doubting you, just curious)

 

[CharlieB]

Re: Do the cylinders on a '71 115 have factory taper?

That's why we stress the FACTORY Service Manuals
Check here http://forums.iboats.com/showthread.php?t=309079

Don't waste your time with Seloc or Clymers.

 

[FoolOfAFisherman]

Re: Do the cylinders on a '71 115 have factory taper?

I can see why now. That's disturbing to see how big a difference there is between them. I'll definitely pick one up. Thanks for all the info guys.