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PostPosted: 14 Apr 2018, 21:57 
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
Do me a favor. With the head off the block I want you to turn the crank until a piston is part way on the down stroke. Take a tool or your fingers and, while holding the crank in position, push the piston down and see if there is a lot of movement.


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PostPosted: 14 Apr 2018, 21:59 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
Raining cats and dogs right now, but I'll do that in the morning when the rain let's up.


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PostPosted: 15 Apr 2018, 15:16 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
Pappy, I tried it several times in both cylinders and at varying positions in the down stroke and could not detect any piston movement in either cylinder.


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PostPosted: 15 Apr 2018, 16:14 
Donor
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
Okay. That vintage engine had a bit of a history in the wrist pin area. Pilot holes in the pistons would go oblong allowing the wrist pins to loosen. Thought that may have been a contributor to low compression.
If rebuilding that is good to know that they are apparently in good shape.


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PostPosted: 15 Apr 2018, 16:34 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
Thanks, Pappy. I'll double check for that when I get the pistons out.

On a side note, these things didn't come out of the factory with helicoil inserts in the spark plug holes, did they?


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PostPosted: 15 Apr 2018, 20:03 
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
No. And Heli-Coil would not be my first choice as a repair.
Am thinking you may want to go to www.aomci.org and post in the "webvertise" section that you are in need of a good powerhead. May be cheaper in the long run. Rebuild yours down the road. Fun project if nothing else.


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PostPosted: 15 Apr 2018, 20:38 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
No hurry, Pappy. I want to make this one work
Stubborn that way, I guess.


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PostPosted: 19 Apr 2018, 21:33 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
I've read that the carbon oil seals at the top and bottom of the crankshaft are virtually indestructible and do not require changing unless there is obvious damage
Any truth to that?


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PostPosted: 19 Apr 2018, 22:00 
Donor
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
Yep....and you cannot get them anymore as far as I know either so take care of them. If I were you I would look up the number for oversize pistons and bore the block for them instead of just honing and re-ring. Source pistons through ebay or ??.


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PostPosted: 20 Apr 2018, 18:00 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
Thanks for the info, Pappy.

I thought about the oversized piston option, but evidently you can not bore out the 10 horse cylinders. It also appears from the manual that there are not and never have been oversized pistons for this motor. I am finding used standard sized pistons on eBay, but have no reason to believe they are any better condition then the ones I have. The cylinder bores mic out at exactly factory specs, but the current pistons are .002 over maximum piston-to-cylinder clearance so I would seriously consider boring if I could find the oversized parts.


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PostPosted: 20 Apr 2018, 19:19 

Joined: 04 Feb 2013, 22:11
Posts: 429
macawman wrote:
Thanks for the info, Pappy.

I thought about the oversized piston option, but evidently you can not bore out the 10 horse cylinders. It also appears from the manual that there are not and never have been oversized pistons for this motor. I am finding used standard sized pistons on eBay, but have no reason to believe they are any better condition then the ones I have. The cylinder bores mic out at exactly factory specs, but the current pistons are .002 over maximum piston-to-cylinder clearance so I would seriously consider boring if I could find the oversized parts.
Do you have a dial bore indicator so you can check for being out of round? That will give you a fit. I find it hard to believe rings could being so worn out and the bore wind up being ok.


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PostPosted: 20 Apr 2018, 19:55 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
No, I do not have a bore indicator. I used a digital inside micrometer and took multiple readings at 45 degree intervals. I'm thinking the rings were not that bad. They were all free. Additional, the first ring groove had the expected carbon accumulation. The second had relatively little and the 3rd groove had none at all. That leads me to believe the rings were doing a pretty fair job.


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PostPosted: 20 Apr 2018, 22:09 

Joined: 04 Feb 2013, 22:11
Posts: 429
macawman wrote:
No, I do not have a bore indicator. I used a digital inside micrometer and took multiple readings at 45 degree intervals. I'm thinking the rings were not that bad. They were all free. Additional, the first ring groove had the expected carbon accumulation. The second had relatively little and the 3rd groove had none at all. That leads me to believe the rings were doing a pretty fair job.
What was the end gap on them then? That should tell quite a bit of the story. If they were not stuck, not a great deal of carbon and the cylinders are within spec. you shouldn't have had such poor compression. There has to be an underlying problem somewhere or a combination of problems of components right at the edge of being right at out of spec to cause it to have such poor compression.


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PostPosted: 20 Apr 2018, 22:29 
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
Search should be on for NOS pistons then.
That was the main reason I asked about any looseness when the piston was on the down stroke. Now that you have the pistons out what does the wrist pin to bore fit look like? Should not be loose at all. Also, take a cylinder bore measurement on the "bridge" between the exhaust ports then above and below the ports. Curious to see if the material around the ports is bowed out a bit.


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PostPosted: 21 Apr 2018, 09:02 

Joined: 24 Feb 2018, 18:32
Posts: 152
Location: Huntsville, Al.
Nccatfisher, now that I have the motor torn down, I'm finding mostly gasket and seal issues. There are signs of real problems with both the head and crankcase cover gaskets. The lower crankcase seal is also cracked. It does appear now that some of the parts are at the edge of third tolerances, but I'm pretty sure gaskets are the main issue.


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