70 hp merc rebuild

dms · Apr 8, 2006

I burnt the top piston on my 82' 70hp 3 piston merc last fall. I am taking it apart this week. How can I tell if it's too far gone for a rebuild?

AMD Rules · Apr 9, 2006

Re: 70 hp merc rebuild

Once you open it up, you need to examine the condition of the cylinder wall to evaluate the extent of the damage. That cylinder cannot be re-sleeved, but it can be machined up to .030" oversize if necessary. If the scoring is light, you may be able to simply hone it. Aluminum deposits on the bore can/must be removed with muriatic acid before honing. While you have it apart, it is a good time to replace the crankshaft seals and inspect the bearings, reeds, carbs, etc. Most important, you should be confident that you know the reason the piston burnt up in the first place, to make sure you don't do it again and fix any of the reason(s) why it did. Have a peek at the links in my sig for info about my 60hp rebuild process. Good luck.

dms · Apr 9, 2006

Re: 70 hp merc rebuild

Thanks AMD Rules, I am relatively sure that the needle in the top carb was stuck, and the top cylinder and probably half of #2 was running too lean. I have always had problems with that damn needle getting stuck in the past. I've taken the carbs apart many times and rebuilt them once, although I don't believe the needles came in the rebuild kit. Even though I use fuel stabil I still get stuck needles after the boat has been sitting in the off season. I plan on rebuilding the carbs again, complete with new needles this time, and draining all carbs of fuel manually on last use of season. Can I position a caliper in what I consider to be the deepest scratch in the cylinder wall and tell if it is worth boring/honing? Or do I just have to get it bored to the oversize specifications and then see how it looks? Thanks for your help. I'll be checking out your link.

dms · Apr 12, 2006

Re: 70 hp merc rebuild

I'm having one hell of a time trying to get the flywheel hub off the spline of the crankshaft. I've broken one puller already. I've been spraying it with some wd and tapping it for two days now. I'll give the puller some good tork tommorrow, if I break this one I will be able to get another one before good friday.

AMD Rules · Apr 12, 2006

Re: 70 hp merc rebuild

Don't hit the flywheel though.... assuming you are only tapping the top of the puller center bolt as you apply pressure. If it has been a long time since it has been removed, you're in for a treat. They're a #itch to get off sometimes. Good luck. Merc offers a puller design specifically for that purpose. I've used both. What type of puller are you using?

dms · Apr 13, 2006

Re: 70 hp merc rebuild

It's the hub that the flywheel bolts onto that I'm trying to get off, it's about 2 1/2 inches in diameter. I have the flywheel already removed from the hub. I first used a twin leg puller which broke. Now I'm going to try a small 3 jaw gear puller that seems to fit quite nicely, however it's from Princess Auto so I'm not sure about the quality.

rosedale · Apr 14, 2006

Re: 70 hp merc rebuild

Wow I had a 70 3 cycinder that did the same thing to me (3 times) I melted the top piston then they put a high volume pump and a popit valve in and that fixed it. what was happening was the motor ran cool at a idle but when you had it out under power it would not get enough water to the top piston to cool it causing it to melt . I highly stress you do the same. My motor came with NO popit valve and Iam sure yours may be the same . High volume pump and popit valve is the way to go . Just thought I would give you my 2 cents worth, I hate to see you go threw the same thing as I did

dms · Apr 15, 2006

Re: 70 hp merc rebuild

Thanks for your input BC-boater, I'll definitely look into that. What year was your motor? I called the mercury authorized dealer for the proper flywheel remover and they wanted 130.00 for it. But the guy was cool and said it I brought it over to him he would pull the flywheel for 10 bucks!!

rosedale · Apr 15, 2006

Re: 70 hp merc rebuild

DMS my motor was a 79 or a 80 and I had it on the back of my 16" glasply, It moved the boat good I was very happy with the power it had till I kept melting pistons . after I had it fixed for the third time I ran it a few times at the lake then sold the boat

Boatwizard · Apr 16, 2006

Re: 70 hp merc rebuild

If you rebuild it put a 50 hp piston in the top hole and 70 hp in 2 and 3. It will idle a ton better and less chance of burning number one again. Some folks use 50 hp pistons in all three holes.

AMD Rules · Apr 16, 2006

Re: 70 hp merc rebuild

Wizard, I'm curious to know more about the 50hp piston theory. What is the difference, and what is the advantage? dms, you get that flywheel off yet?

dms · Apr 16, 2006

Re: 70 hp merc rebuild

Tommorow's Monday and I'm hoping to bring my powerhead to the shop so they can get that damn flywheel off for me. I can't wait to see inside the cylinder wall for damage. I don't have the expertise on how much scoring is too much. I'll take it to Outboard Rebore here in Winnipeg and see what they have to say. AMD Rules, You mentioned I could go to a .030" oversize. From some of the websites it looks like I can go to a .040" as well. Is that possible? This is the link to the site. 1982 3cyl 70 h.p http://www.maxrules.com/fixphead.html

dms · Apr 16, 2006

Re: 70 hp merc rebuild

AMD Rules, By the way your rebuild blog was very helpful and interesting. Crazy times you had there. Are you still running that motor?

dms · Apr 17, 2006

Re: 70 hp merc rebuild

O.K. I finally had the flywheel removed and was able to open that puppy up today. It seems there is good news and bad news from what I can tell. The good news is that the cylinder walls hardly look scored at all. I wish I knew how to post pictures here cause I snapped quite a few. The bad news is that there is a small crater for lack of a better term, in the #1 cylinder wall. It's about 1/8" in diameter and about 1/16" deep. It is located about an inch below the exaust ports. How bad is that? Am I going to the scrap yard with this thing or what? I can email pictures if anybody is interested.

AMD Rules · Apr 17, 2006

Re: 70 hp merc rebuild

Is the 'crater' in the path of the rings during normal operation?

dms · Apr 17, 2006

Re: 70 hp merc rebuild

Good question, I'll have to check the stroke and get back to you.

dms · Apr 17, 2006

Re: 70 hp merc rebuild

Just went out to my shop and dropped the crank back in and from what I can tell the last ring comes within an 1/8" from the 'crater'. I assume that would mean I'm in good shape. The other two cylinders have a small machined hole about 1/8", below the exaust holes about 1/2" what are those holes there for? Curious.

dms · Apr 19, 2006

Re: 70 hp merc rebuild

Well, I've decided to throw in the towel. I'm getting Outboard Rebore to rebuild my powerhead for me. Thanks for all your help AMD Rules.

AMD Rules · Apr 20, 2006

Re: 70 hp merc rebuild

Let me know how you make out. I've heard they do good work. Good luck.

dms · Apr 20, 2006

Re: 70 hp merc rebuild

Thanks AMD Rules, By the time I order all the right tools for the job and parts everybody will be out on the lake but me. I should have started this a long time ago if I wanted to rebuild it myself. I might hit you up with some timing questions, when I get it back. I guess in a new posting though. Later

70 hp merc rebuild

Cadet

Lieutenant Commander

Cadet

Cadet

Lieutenant Commander

Cadet

Recruit

Cadet

Recruit

Seaman

Lieutenant Commander

Cadet

Cadet

Cadet

Lieutenant Commander

Cadet

Cadet

Cadet

Lieutenant Commander

Cadet