Cylinder Wear

[Jho928]

Hi ,I have a Evinrude 25930 35 hp '57.My question is...How much cylinder wear is acceptable? .My manual states bore spec as 3.0625".Now is there a +/- tolerance here .I measured a couple spots and got 3.0615" +/-.I guess to put it simply at what point do you rebore? Say .005" over spec or is this spec the max?Thanks

 

[Paul Moir]

Re: Cylinder Wear

I find the best way to do this is to measure the piston to cylinder clearance, and the piston ring end gap of a new ring, since these are really what's important. <br />1) Put a new ring in the bore, and push it in partway with the end of a piston, so it's in there square. Then measure the end-gap with a feeler guage. 0.017 max, 0.007 min<br />2) Put the piston in the bore and measure the piston to wall gap with a feeler wire. You must check this at the skirt (bottom) of the piston, since they're tapered. 0.0045 max and .0030 min.<br />Alternative 2) Measure the bore and measure the piston at the skirt. Subtract to get your clearance.<br /><br />Hope this helps!

 

[rodbolt]

Re: Cylinder Wear

hello<br /> bore size standard is 3.o625 ?<br /> now it is 3.0615?<br /> man this motor is getting better with age sounds like its close enough. that old 3 ring piston will work well up to .004 piston to wall clearance. I also use the measure the piston at the bottom of the skirt 90 degrees from the wristpin the subtract the piston measurement from the bore and you will get piston to wall.if the pistons are not heat damaged break the glaze with a hone very carefully and reuse it.<br /> always use new rodbolts. reuse the rod bearings and wristpins if ya must but allways use new rodbolts.<br /> good luck and keep posting

 

[Jho928]

Re: Cylinder Wear

Hi,Thanks for the info,The problem is I don't have new pistons or rings.I don't want to buy rings and pistons and then find out I have to bore.I could use the old ones but if my cylinder is worn I would guess the rings and possibly piston are worn too,therefore not giving me accurate measurements.The Spec is 3.0625<br />I'd imagine there is leeway here .It is unknown to me what that is.I'm sure every bore is not 3.0625 exactly.So after 40 years of wear and after honing the cylinder will be enlarged.Say for example I wind up with 3.0655 +/- .Thats .003 over Spec,but is it ok for standard piston and rings?

 

[Paul Moir]

Re: Cylinder Wear

What size do your pistons measured, as rodbolt says, 90° from the wristpin at the skirt?<br />If your into replacing pistons for whatever reason, you might as well go to oversized.

 

[rodbolt]

Re: Cylinder Wear

hello<br /> I really dont think you can still get new pistons from boby for a 57 35. so I would carefully hone the cylinders taking care that you are only trying to deglaze it not remove stock. then measure the bore measure the piston. subtract piston diameter from measured bore diameter. if the difference is less than .oo5 run it. remember we are looking for a max piston to wall clearence. those old 3 ring pistons not only transferedd a lot of heat they seal very well. but more rings add drag and eat horsepower but more importantly drive up the machining costs. remember in 1957 cnc machining was a pipe dream and piston castings were machined with manual equipment. if the old piston skirts are not scuffed your fine. I have a 58 or so power head left with the compression release head that I may let go, hasnt run in at least 14 years. sold off all but the head assy. anyway I will tell ya if you have no ridge at the top of the piston ring travel odds are your wear is negligable. two strokes just dont wear that much. if this was a big v6 I would tell ya to bore all cyl and replace all pistons. but were talking about an over designed 57 model 35. that motor will run when its wore slam out and the pistons are slapping.<br /> we ran the old 35 and mechanical shift 40 seahorses in the ocean for beach launched net dory's. best motors ever made. always came ashore. would run on 1 cyl with a blown head gasket if ya asked it to.<br /> so inspect everything and if the tolerances are close then run it.<br />good luck and keep posting

 

[Jho928]

Re: Cylinder Wear

Thanks ,I'll lightly hone,then measure my bore and piston(at the skirt).I'll hope for a clearance under .005 (Spec is .0035-.0040).I should do this next weekend I'll post results.Thanks Jim

 

[CFronzek]

Re: Cylinder Wear

Two-stroke engines are much more sensitive to cylinder/ring wear than their four-stroke counterparts. <br />Even if a two-stroke will run with slappy pistons and shot rings it will be way down on power. <br />This is because both sides (top and bottom) of the pistons are involved in making power. As cylinder/piston/ring wear takes place combustion gasses get past the piston/rings and dilute the incoming charge, which leans the incoming charge out and makes for power loss and hotter running. <br />A lot of two-stroke engines have replacement pistons in .010" increments so that power output can be maintained.<br />The bottom line is a bore job with new pistons and rings freshens motors up to where they were when new. <br />It's worth doing if you can afford the price.<br />Charlie