Logo
FULL SITE MENU

Low compression

That top ring is the problem-----As the piston / ring / cylinder wear the side clearance INCREASES and the ring is loose in there.-----At 5000 + RPM it breaks !!----New pistons from WISECO are one solution.
 
Just finished disassembling. #1 cylinder wall feels scuffed worse than I thought, but I'll let machine shop decide what to do. I may have to send to Knoxville if it needs re-sleeving. No one here local does sleeving jobs.View attachment 20034View attachment 20035 What are these pits in the exhaust ports? Is this any thing to worry about? Should I request the machine shop to dip the block in cleaning tank, or do they do that any how? I will be assembling this motor myself, I hope.

I hope I'm not wearing out my welcome.
Roy
 
racerone said:
That top ring is the problem-----As the piston / ring / cylinder wear the side clearance INCREASES and the ring is loose in there.-----At 5000 + RPM it breaks !!----New pistons from WISECO are one solution.
Click to expand...

Yes I will be replacing pistons and rings.
Thank you for advice about the WISECO brand
 
racerone said:
That top ring is the problem-----As the piston / ring / cylinder wear the side clearance INCREASES and the ring is loose in there.-----At 5000 + RPM it breaks !!----New pistons from WISECO are one solution.
Click to expand...

In agreement about WISECO... top quality... used them countless times.
 
Send to :
Jay’s Block Shop
3015 McMinnville Hwy
Sparta,Tn. 38583 (931) 808-0814​
Jason will fix it up..tell him Alan sent ya. Wiseco's are OK if you break em in and let em harden, I use cast pistons like OEM (WSM)
 
Joe, Racer, Pappy, thanks for the education. These tapered rings, close to the top of the piston are fine in the perfect world, but one mishap will cause what we see here. Once again demonstrated......innovations and design "improvements" sometimes backfire. If I wanted to build compression, I could probably think of some safer and more reliable ways.
 
Its a good design by engineering spec and in a perfect world. Problem is carbon builds in ring land hammering it, this loosens the tolerance and ring rolls and breaks. The carbon is formed by cooling problems,cheap oil, cheap fuel,incorrect RPM and general lack of maintenance and correct operation. I have a customer that has a 1976 BMC bassboat with matching 85 Johnson, that he bought form the dealer I worked at. It looks like its only 1-2 years old, is used 3-4 time a week,has NEVER had carbs rebuilt only fuel lines replaced, lower unit seals once and impeller yearly. Runs OEM oil and its measured meticulously. Runs as good as day it was purchased.
 
Pappy, I had a 76 85 Johnson also. Loved it. Sold it several years ago. Original and running perfectly. Like you demonstrate here, properly take care, but don't mess up or you'll pay for it dearly. Ignorance might be an excuse, but it sure won't save your money or your equipment.
 
Last edited:
I had a 1978 85hp johnson last 85etlr78c . It finally gave way last year. I owned for the past 15 years or so. Very dependable, and I used it alot.

I am now looking at the manual for that old johnson. It is very similar to the evinrude I now have. The only differences I see is the evinrude has what looks like vacuum lines running from intake to engine covers,? I would guess these are oil lines? And this newer motor have a different power pack. The old johnson had 2 power packs.

Taking block to machine shop in the morning. If I was closer to Sparta I would take it to Jays. Should I clean the block up before I take it? And what should I use to clean it and other engine parts? If it can be rebored, I will start buying parts for.

You all have a great Easter, and thank you.
 
I would consider the 88 HP model a better version of your 1978 85 HP.-----Bigger cemter main bearing.----And more displacement.------Electrical bits work the same way.----Has the Electric primer valve instead of choke flappers----Should be an easy fix.
 
2019-04-24 21.46.53.jpg I just got the engine block from machine shop. Had to bore .030 to clean up cylinder. Getting ready to order complete kit with over size pistons, all bearings, seals, rod bolts and gaskets. I found a good used head for 80 bucks. After the carb kit, thermostats and water pump kit I should have around 1,200 or 1,300 in it.

Any last words of wisdom?
 
Make sure you have proper piston to bore clearance. I always recommend supplying a new piston to the machinist for final hone/fit.
 
Ooh I just ordered pistons. .030 OS I guess if it isn't right I'll take block back to him and have cylinder honed out some more. I didn't think of that. Thanks Timguy.
 
Modern manufacturing / tools means a 0.030" O/S piston fits with proper clearance in a bore that has been opened to 3.530"----Use the factory " gel seal " on the metal to metal joint.-----Read up on the correct procedure to install / align rod caps and bolts.---Bolts are dipped in oil before installing.
 
Thanks Racer. At a 3.5 inch bore I need .004 clearance or even .0045, or I'm not going to assemble it. You are probably gonna find that's what it will end up, but I have been burned by machinists before, so I always tell them what I need and then double check. On a 2 stroke outboard.....and you must know this very well, the piston temperatures run hotter when compared to that water cooled block with 145 degree regulation. On 4 strokes the fire is every other stroke so piston temps typically run cooler, more in line with block temps. In other words, the ratio of piston to block temps are different between water cooled 2 strokes and water cooled 4 strokes, thereby naturally directing us to calculate adequate clearance accordingly. You can tighten the clearance to .001 for each inch in bore size, for 4 strokes I owned and managed a machine shop for 2 years, employing 8 guys. Found out I didn't like it.
 
Last edited:
Yes, I'm making it too complicated, but explaining how the clearance should.....and probably is, calculated. I have a good set of micrometers and always double check. If you don't have a micrometer, you can check with a standard envelope and cut a strip about 1 inch wide by 4 or 5 inches long. Have your block and piston at about room temperature and slide the piston into the bore with that strip of paper on piston skirt running 90 degrees to the rings. Should fit snug but still slide the full stroke where the piston will travel. Make sure ports are not sharp and that they have been lightly radiused. This will ensure your machinist has done his job. So you will have your .004 clearance. Check all cylinders to be sure.
 
Last edited:
Yes a few of the ports have sharp edges. What is best way to smooth these? I can barely get my big ole meat hook of a hand in there.
 
3M wet or dry with 360/400 grit. Use water with a little soap. I get my wife or son to do it. I have big mitts too. Most shops don't like that job either.
 
I got the ports edges smoothed over, along with a couple of cut knuckles. I received the engine kit today and I checked the piston to cylinder clearance with the paper strip that Timguy suggested. 2019-05-01 15.16.27.jpg the kit came with ThreeBond 1184. 2019-05-01 20.17.50.jpgIs this to seal crank case to block, or should I find the gel seal that Racerone said to use?
 
Roy, never used it myself so I am not going to advise. I use a Locktite product. Clearance looks good too there. Ouch on the knuckles. Did you put masking tape on the top cylinder edge while you were working......NO. Forgot to remind you, brother, my bad.
 
Last edited:
I do like the grafal coated pistons. We do that here where I work. 2019-05-01 22.52.02.jpg It is a great antifriction coating. Just about all the NasCar pistons get it. We put that coating on all the Ford, Chrysler, Toyota, Polaris, steel heavy duty truck pistons and Mercury Marine pistons. If the parts are processed properly before the coating is applied, it will not come off. It will wear down on the skirt center vertically from the up and down stroking. If the coating comes off and scars my cylinder walls, me and the supplier that I purchased them from are going to have a conversation. Sorry I'm rambling on.

Any how I took a vacation day, so I don't go back to work till Sunday night. Maybe I'll make some good progress on this assembly.
 
Right, of course they are placed exactly into position to direct water circulation. I like those pistons. Are the rings chamferred to increase cylinder wall pressure? To me, that theory is like a double edge sword. Like dah!.....why is that top ring so far down?
 
timguy said:
Right, of course they are placed exactly into position to direct water circulation. I like those pistons. Are the rings chamferred to increase cylinder wall pressure? To me, that theory is like a double edge sword. Like dah!.....why is that top ring so far down?
Click to expand...

As long as their coating adhesion is good, they should work nicely. Yes the top ring is chamfered. With the top ring being lower maybe reduces chance of possible top land breakage? I haven't matched them to the pistons I removed, yet. So I could see the difference, if any. The older 85hp Johnson that I took apart last year was strange that, there was a lot of distance between pistons and cylinder wall. I had the motor for 15 years, and it ran great. But when I removed the head I could move piston side to side in cylinder, like someone bored it and put over size rings but didn't change pistons?? I could still see the cross hatched hone marks.
 
Dem pistons look like Siezecos...If so make sure you put them thru several several heat cycles to temper em up as da soft as butter new.
 
Back
Top