Expert opinions/data on piston ring gap and wear 1990 V4 90hp Johnson

[havoc_squad]

This is asking for professional viewpoint on wear limits/tolerances based on the published ring gap maximum for the piston rings of a 1990 V4 90hp Johnson.

Data:

All cylinders were checked by machine shop to be within specifications for standard bore size per the OEM manual I gave them to check. I was not able to get the exact measurements, even though I did ask beforehand. (I didn't want to make a problematic situation worse, there's not many machine shops around locally.)

Cylinder 1
Compression ring gap: 0.032
Oil ring gap: 0.032

Cylinder 2 (had stuck oil ring)
Compression ring gap: 0.033-0.035 (hard to discern this one)
Oil ring gap: 0.027 (double checked but may revisit this measurement again)

Cylinder 3
Compression ring gap: 0.031-0.032
Oil ring gap: 0.031-0.032

Cylinder 4 (had stuck oil ring)
Compression ring gap: 0.032
Oil ring gap: 0.031


My summary: It appears based on ring gap max specifications, the piston rings are right at the beginning of the downhill wear. which means as the hours progress on the motor, the compression loss and oil blowby will greatly increase.

It looks like I'm going to be in a market for a piston assembly on all cylinders. Unfortunately I haven't gotten any response on previous questions on the pros and cons of OEM versus Wiesco pistons in OMC crossflow outboards.


My questions: What should be the average expected ring gap on a new piston assembly for a used standard bore within spec? (Likely a hard or impossible number, but asking in case someone has done enough to build a spitball average.)

For a used bore, would 0.005 below max ring gap tolerance be the expected/average?

 

[flyingscott]

There is.no oil ring on that motor. How are you measuring ring gaps? Hard to believe you have a in spec stock bore. Unless your motor is very low hrs.

 

[racerone]

There is no oil ring on that motor !!!------There is no concern with " oil blow by " on that motor.----0.032" ring gap is too high !-----Do not fit new rings on worn pistons !!-----I am going to be blunt once again by saying that oil / lubrication on a 2 stroke engine is not well understood !!----Post some pictures of your pistons.----Do you know what your pistons measure at the bottom of the skirt?----In line with the wrist pin and at 90 degrees to that ?

 

[havoc_squad]

Regarding the statement of no oil ring exists. By the oil ring I meant the bottom ring, I was not referring to the normal four stroke oil ring.

Feeler gauge. I go 0.035 and check (+0.004 max). If it won't fit in the gap or moves the rings, I go 0.001 down at a time and test. Making sure the piston ring doesn't move and it slides in. Repeat going down 0.001 if resistance is still felt.

I would say if there is any margin of error, I would say at worst my feeler gauge measurements are +/- 0.001 to +/-0.002 of what I have recorded.

Yes, I am assuming this is a relatively lower hour motor from previous owner's use history and condition.

Yes, I am aware of the need to replace the whole piston assembly and cannot re-ring the pistons, that is why I asked about new OEM versus Wiesco piston kits.


Videos of each piston, rotated around for 360 degree view.

Piston #1 video: https://youtu.be/ltmnd8UZNPg

Piston #2 video: https://youtu.be/W0-3peKjo6M

Piston #3 video: https://youtu.be/4oTYQrWfYJc

Piston #4 video: https://youtu.be/fRoMxoHAN-o


I will get piston skirt measurements data soon as I can get me a micrometer to check that. Dial calipers would be a bad idea on this.

 

[racerone]

And measure the piston diameter just above the top piston ring too.------Measure where you see the wear on those wee grooves.----I suggest 4 new pistons and rings.----Wiseco is a good option.

 

[havoc_squad]

Would it be a good idea to just order one new Standard piston assembly to first check ring gap on all of cylinders with the new kit to rule out any discrepancies before ordering the rest of the other piston kits since all are standard size holes?

That way if there was a breakdown in the data of the machinist or my ring gap measurements, I only have to pay a return fee penalty (that's if return policies allow just checking ring gap is being "not used".) or eat the cost of one piston kit.

 

[Faztbullet]

Are you measuring new rings in the cylinders??? If so the measurement post are for a worn out cylinder as .030+ is way to much. .015-.017 is perfect.

 

[havoc_squad]

Are you measuring new rings in the cylinders??? If so the measurement post are for a worn out cylinder as .030+ is way to much. .015-.017 is perfect.

No, I measured the existing/old piston ring end gap before I go and spend any money on parts. I was checking to see if I needed to replace the pistons or not. Racerone confirmed my initial conclusions along with the factory manual data.

Since ring end gap measurements are over spec, that means I need new piston kits since the bores were checked by machine shop to be within spec for standard bore size of the factory service manual for out of round, taper, total wear, etc. (that manual was provided to them).

Overall steps taken

1. Piston ring gap check (all are out of spec)
2. Check cylinders since ring gap is out of spec (machine shop validated cylinders are in spec of standard bore size of provided factory manual tolerances)
3. Decide on piston kits

 

[Faztbullet]

I would ask shop that checked what size was...if 3.504 or bigger they are worn. should be on ticket or wrote on block with paint pencil. That how a good shop would do it

 

[havoc_squad]

If I have to hunt down another machinist for me to accurate data, I'm going to need to hit the pause button for a month or two on this.

As mentioned above, I tried to get that measurement data but could not from the one that did the measurements.

I'm not to going throw parts at it, nor am I going to hastily find the nearest outboard machine shop to send it to. I don't need a short turnaround on this now, not to mention their queue is very long right now (peak season).

If one is going to ship it sight unseen and not be able to visit if there is a problem, where can one get a list of the most reputable OMC outboard machinists in the continental US?

I've already spent money to have it checked once, I really don't want to waste +$100 in two way shipping to have bad data repeated.

The only alternative I can think of is spending coin for a dial bore gauge and micrometer of sufficient quality for at least one or two jobs.

Bad data on major buying decisions really leaves a bitter taste as a working professional on projects/jobs because most likely, you have to eat the cost of it.

 

[racerone]

You do not need an outboard shop to do these measurements.-----Tried your local garage ( do they exist anymore ) or a small engine shop ?----Looked into tool rental / loan possibilities.----High school machine shop ( if they still exist ) for help.----Your location ?

 

[havoc_squad]

I was fortunate enough to find another machine shop in short time nearby that was able to work it in. I dropped it off this early morning and they got me the data a few minutes ago over the phone.

They said out of the +0.004 max spec, all the cylinders were right in the middle (+0.002) of standard bore 3.5 inches.

I mentioned for them to please have that data written down and I will pick it up early next week.