Home › Forum › Ask A Member › Top speed with 18hp Evinrude on 14′ aluminum
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stephenspann27.
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January 12, 2017 at 9:27 pm #50907
Looks like I can get .40 over pistons for cheaper than stock if the bore is bad.
January 12, 2017 at 10:04 pm #50908quote fleetwin:Well, you certainly did a nice paint job.
Like others have said, compression reports seem to vary, but I would think you should see 100PSI minimum. Why is it that you replaced the original powerhead? You also seem to say that the engine ran the same way with its original powerhead, do I have it right? You mention seeing score marks on pistons of both powerheads, needless to say these could be an issue, or not. Most two stroke OMCs will show some minor scratching/scoring on the pistions, usually not an issue. But, if there has been aluminum transfer, and the rings are stuck in place that is another story. You mention the engine starts easily and idles nicely, which suggests the powerhead is "OK".
You went through a lot of work cosmetically, would be a shame to mess that up over something simple. I guess I would start with something simple like having a look at the mag plate and carb linkage. Is the mag plate advancing fully, perhaps the interlock is interfering. Is the carb butterfly opening fully? Is there a chance that someone swapped out the carb from a 25hp, thinking this would make the engine run faster. Perhaps they just changed the high speed jet, thinking this would be the answer to their performance dreams. I would mention a blocked exhaust, but I’m thinking you would have seen a problem like that when you had the midsection apart.
Don’t forget to check the prop pitch, the engine will just bog down if it is over propped.
Tell us more about the history of this engine, why you changed the powerhead, and how it ran with the old powerhead. I don’t want to mislead you based on my “assumptions”.Honest, these engines will start and have have low end like an absolute dream but won’t make their best RPM, with lower compression. I can’t even imagine how many 18s I have pieced together over the years and have seen the results of reassembly with new rings only, vs. a TRUE rebuild. If you take it apart, check the bore or at least the ring end gap. I bet you will find the gaps too large. The bores wear out.
Long live American manufacturing!
January 12, 2017 at 10:23 pm #50910Yup been there done that Bill!
I just rebuilt an 18hp powerhead for someone with similar situation.
I gave him a quote which he thought was high. Brought it to someone else. THey simply put new rings in, and sanded the bore with sandpaper. Called that a rebuild. I ended up getting it back after he could not get it to run after the rebuild at high speed.
When I got it apart, found the bores out of round, and oversize to boot. All bearings shot.
You get what you pay for, go figure my grandpa was right.
January 12, 2017 at 10:40 pm #50911You probably know this, but if you are running it as shown in that photo where it’s clamped to the boat, you need to raise the trim pin a couple notches to cut boat drag and you will gain a few MPH, it will just take longer to plane off.
DaveJanuary 12, 2017 at 10:44 pm #50912Stephen,
My best advice is take your time, measure, inspect, and play detective.
Start with the bore. It is first within spec? Not out of round? Tapered?
The bore should be 2.496" Opinions on out of round tolerances or wear will vary.
Your piston to cylinder wall gap should be within .0035 to .004. Mic your piston OD, then subtract that from the Bore.
The ring end gap by the book is .007 to .017, however any more than .014 and I re ring for sure.
Once you determine if your block is good (always step #1), move on to your pistons. Measure OD. Inspect side wall for grooves. Inspect wrist pin bore.
Check the rods. MIC the ID of both big end and small end bores. Inspect for damage or grooves, or signs of overheating.
Needle bearing re use will get varying opinions. If they are bluing, absolutely toss em. Inspect them with a magnifying glass. When I do a rebuild, to be honest I install new. I figure if they got through 75 or so years, they have done their job.
Crank bearings are another tricky one to "test". Again, if I am going through trouble of a full rebuild, I install new bearings on the crank.
Check the crank halves, ensure they are true. Clean up spotless. Obviously install a new spaghetti seal. Use the proper primer and anaerobic sealer. Silly to go through all this, and pump silicone into it. Don’t laugh, seen it done. Install new gaskets, and ensure gasket surfaces are clean and true. Change ALL the seals.
Then, when complete, ensure you properly break it in. We can help with that. When you are done, if done properly, will last another 100 years, for your kids, and your kids-kids, to enjoy.
As you can imagine, doing it right is definitely NOT cheap. Yet rewarding.
January 12, 2017 at 11:05 pm #50915Here are some photos of the pistons and the bore of the original powerhead that I tore apart. The pistons look worse than what I have been able to see on the current powerhead but all of the rings are free, not chipped, there are some light scratches in the rings but nothing bad. This by no means, means that my current powerhead is fine, but it gives me some hope that maybe it’s just a prop thing.
January 12, 2017 at 11:06 pm #50916quote outbdnut2:You probably know this, but if you are running it as shown in that photo where it’s clamped to the boat, you need to raise the trim pin a couple notches to cut boat drag and you will gain a few MPH, it will just take longer to plane off.
Daveyes sir I’ve played with different trim positions
January 12, 2017 at 11:10 pm #50918quote Chris_P:Stephen,My best advice is take your time, measure, inspect, and play detective.
Start with the bore. It is first within spec? Not out of round? Tapered?
The bore should be 2.496″ Opinions on out of round tolerances or wear will vary.
Your piston to cylinder wall gap should be within .0035 to .004. Mic your piston OD, then subtract that from the Bore.
The ring end gap by the book is .007 to .017, however any more than .014 and I re ring for sure.
Once you determine if your block is good (always step #1), move on to your pistons. Measure OD. Inspect side wall for grooves. Inspect wrist pin bore.
Check the rods. MIC the ID of both big end and small end bores. Inspect for damage or grooves, or signs of overheating.
Needle bearing re use will get varying opinions. If they are bluing, absolutely toss em. Inspect them with a magnifying glass. When I do a rebuild, to be honest I install new. I figure if they got through 75 or so years, they have done their job.
Crank bearings are another tricky one to “test”. Again, if I am going through trouble of a full rebuild, I install new bearings on the crank.
Check the crank halves, ensure they are true. Clean up spotless. Obviously install a new spaghetti seal. Use the proper primer and anaerobic sealer. Silly to go through all this, and pump silicone into it. Don’t laugh, seen it done. Install new gaskets, and ensure gasket surfaces are clean and true. Change ALL the seals.
Then, when complete, ensure you properly break it in. We can help with that. When you are done, if done properly, will last another 100 years, for your kids, and your kids-kids, to enjoy.
As you can imagine, doing it right is definitely NOT cheap. Yet rewarding.
A lot of good information in your post. I might check the ring gap on my old powerhead just for giggles, it only had 90 PSI of compression.
January 12, 2017 at 11:18 pm #50919Obviously those pistons were junk!
In the bore, are my eyes playing trick, or are there gouges out of them? Bottom of 4th pic. OR is that just heavy pitting?
January 12, 2017 at 11:23 pm #50920I am surprised it showed 90 with that bore and pistons.
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