"Newer" 25/35hp OMC compatibilities

[dan-in-tn]

My opinion on this baffle plate may differ from many. While many will be warped when removed and be causing the cooling problems you are experiencing, that is not the way your problem began. IMHO. These motors have a history of many ways to over heat. It takes an overheat to warp the plate. Once that happens you are headed down a road to continued problems till everything is put right again. You don’t have many options but to straighten the plates now and reuse, but the overheat problem must be cured that caused the plate to warp,in the first place. It is kind of a catch 22! I have taken 32 cu in motors apart that don’t have warped plates. The cooling system has to be maintained on these motors. Not just an impeller!

Dan in TN

[johnyrude200]

It makes sense to me. Whatever causes the 1st overheat, that triggers a warped plate…and exhaust pulses pushing water away from the cooling system backwards.

If that isn’t fixed, then it will just trigger more problems because the motor can overheat again, and who knows what happens next.

This motor had the gearcase off when I bought it. The previous mechanic said it was turned over to him from his customer several years ago because the customer didn’t want to spend the money to have the impeller changed out. Well we all know there is more to that story than just that now that I have been working on it here on my end.

What do you think of the self-manufactured aluminum baffle I made today? I mean, this is going to be a fresh water motor, and aluminum doesn’t melt until ~1200°, and the motor would have to seize up well before the baffle ever got to that temperature.

I just don’t want to reuse those old baffles at this point…yet.

[johnyrude200]

this is the differences, for those who haven’t seen, in the water jackets between pre 78 and post 78 motors. Pretty big difference in rerouting the water. Note the ’78 is my motor, very good compression (135+), runner, trying to solve cooling system issues. You can see the broken off screw in the middle cover orifice, I couldn’t salvage this (tap broke off inside…damn it!), so I ran crankcase sealant/gasket seal/fuel and oil tolerant sealant along this middle ‘wall,’ and then used plenty of OMC gasket sealant everywhere else. Hope this does the trick tomorrow morning when I fire the motor off.

The pic following is off a seized 1979 35hp (typo – should say 79-thru, not 80-thru), although I have a ’85 20hp dead block as well that was seized and the cooling jacket is the same in appearance.

[collectorinspector]

Quote "So I ended up fabricating a new plate from 1/16th aluminum (same thickness as the original baffle)."

My last two cents worth.

Personally I suggest that you do not go down this path…..not even contemplate Aluminium unless you know what are the mechanical properties and exactly what type of aluminium.

With aluminium it is not the temperature that is the problem adjacent to the exhaust ports.

The fault you will find is a seven letter word beginning with the letter "E".

Also the last pics (1978) show no evidence of overheating as the Johnson white paint on the block and head is not even burnt yet. Yes, they can burn paint and still be a runner later with luck. I am so confused now as this post is being edited and chops and changes. No engine plate details yet and no confirmation as to what the serial is on the block medallion.

You have put it back together with Glue? You have to fix the broken threads……..any good machine shop easy!

Also pics of the power head?

Have you actually had the head water jacket plate removed and cleaned all of the in and out passages and is the spring on the thermostat free to relieve pumping pressure? Information seems to be in dribs and drabs so far…… so how can we help with a guess.

SO Sadly

I will just follow this post with no further comment from now on.

B

[johnyrude200]

Collector, it’s a ’78. The info you were asking was already posted earlier in the thread.

The aluminum I used is structural plate aluminum leftover from a fabrication job.

In terms of fixing the threads, I posted twice that cannot be done because the tap broke off in the orifice when I was retapping it.

So unless you know something that can drill through tool steel, I did the best I could with the situation & tools I have.

This has been a long thread, and has gone thrpigh a couple of different topics from others as well, thanks.

[dan-in-tn]

In my opinion you are going to need every fastner you have on the earlier motors! Why do you think OMC went to more on the newer blocks? I don’t think the aluminum plate will be too good because aluminums expansion rate is more than stainless steel. The tap can be gotten out, but requires drastic measure of cutting the side of the hole out and then welding the hole up and re-drilling. Best done with a mill for the re-drill. This can become more than you want to spend unless you just want to try with the block for learning purposes. While it was in the mill (if you went this way) it would be nice to deck the side of the block. Trueness would be nice while you are there. You did look to see if you had sleeve separation while you had the head off? I know you had the head cover off because you said you ran it without a thermostat. You should have piston damage with sleeve separation, but compression on pressure back rings is not a good way to tell. With both the exhaust and intake covers off you should see something. This is a siamese cylinder engine (no water between cylinders) so they score at top and bottom of the pistons. Just something to remember. If it runs good maybe you have gotten away with the past over heats. Sending you some pics of a tri-toon pontoon from FL while at the meet down here.

Dan in TN

[johnyrude200]

As a follow up, aluminum melts at 1200 degrees F. Using the powers of the interwebs, I see stainless has a melting point of 2700 degrees F. So not to beat a dead horse with a stick…how the HELL does a motor get hot enough to warp one of these baffles if the pistons are aluminum.

I mean, I can buy a 12 x 12 0.06 sheet of stainless steel for $22 (that includes shipping), and manufacture these baffles all day long tracing the original, with peace of mind (which I may very well do for anything in 1976-1978 moving forward). I will get by with my aluminum baffle on my personal motor for this year because I know the motor is right, but as a repair person in business, I will NEVER send out a motor that I think will cause the customer headaches anytime soon.

Shouldn’t the motor seize up well before we hit 1200…or even 2700 degrees F???

Granted in an overheat, we have little to no water supply…but I feel as if the heat thrown by the internal combustion and vented out the exhaust ports would melt the pistons before the baffle ever hit those temps.

I will say that I had a 1969 25hp johnson (my 2nd motor ever…5 years ago), overheat 5x, seize, cool off, and start on the 2nd or 3rd pull within a couple of minutes, so I am extremely sensitive to any motor I work on with respect to the cooling system. It actually melted the top piston off, and it still started on the 2nd or 3rd pull, but after 5 seconds of WOT would lose power and overheat again, even with the melted piston. I think I may actually still have that power head.

I am anal about it quite frankly. And that motor pumped tons of water, and it came out of the blubber hole, and out of the T-stat orifice with the cover off. In retrospect it may have been a clog in the tiny hole in the cylinder head….but nonetheless, cooling systems are things I split hairs with. So getting very familiar with the newer versions of these motors are something I am putting aside a lot of time for in the upcoming months.

[dan-in-tn]

Aluminum plate is going to move a lot more than the stainless steel even in a non over heat situation. Aluminum by its very nature expands and shrinks more than steel. About 5 to 1. Exhaust gas breaths on the plate all the time the motor is running. Even with a good cooling system the aluminum will become brittle in a shorter time because of expansion and contraction. Aluminum is cheaper than stainless and motor companies would use the least amount of stainless they could if possible.

Dan in TN

[johnyrude200]

Well looks like Ill be buying some stainless plate and fabricating up some few baffles. More time in labor, but cheaper in materials!

[fleetwin]

what about the NOS SS plate I found you on ebay?

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