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Antique Engine Archives All archived posts from 1999 to 2004 when SmokStak was on EnginAds. This is a read-only board.

Antique Engine Archives

1-1/2 FM ZA compression


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  #1  
Old 03-18-2003, 11:18:40 PM
Al Hettich
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Default 1-1/2 FM ZA compression

I have a 1-1/2 FM ZA spoke flywheel. It looses compression about 1/2 way up on the comp stroke. When I soak the rings with oil it will start, but as they dry out it looses compression and dies. The cylinder is not scored and it honed out rather nicely. It has .010 wear at the top just under the ring travel, and tapers to about .004 at the bottom of the ring travel, and is round within.002. The rings have about .015 end gap in the most worn part of the cylinder, and around .008 side clearance. The piston is not scored or visibly worn. The rings were new, and when I took them out they looked like they were begining to seat in. Is this too much clearance or taper? And do you guys feel I will need to sleeve this engine, I just hate that and am looking for a less invasive fix. Al
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  #2  
Old 03-18-2003, 11:35:20 PM
BobRR
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Default Re: 1-1/2 FM ZA compression

Al Whats the ring land clearance? BobRR
  #3  
Old 03-18-2003, 11:45:33 PM
Al Hettich
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Default Re: 1-1/2 FM ZA compression

Side clearance (ring to piston) is about .008. The ring lands are slightly tapered from wear, the rings set in fairly nice and do not seem to bind. the grooves are worn equally all around but the wear does not appear to be excessive. Al
  #4  
Old 03-19-2003, 12:13:24 AM
Vernon
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Default Re: 1-1/2 FM ZA compression

I've seen cylinders worse then that that would seal. Sounds like the rings where seating in, and then you took them out. They are wore to the way they were before you took them out. You put them back in and now they are in a differant position, so they leak even worse. Case of odd shaped rings in an odd shaped cylinder. You can keep running it hoping the rings will eventually seat, or you can start over with new rings. The cylinder isn't pitted, is it? Only other fix is sleeve or bore bigger.
  #5  
Old 03-19-2003, 12:50:40 AM
Jim Tremble
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Default Re: 1-1/2 FM ZA compression

Al

Did you mark the rings and piston when you took the piston out ??

If so, put it back together and pull the spark plug out. Pony it up to another engine and run it for 2-3 hours, no compression. Oilier on.

Try starting it under it's own after a few hours of running.

I have done this many times on smaller engines with new rings -- old bore -- and have had great sucess. Called "running in"

All of your specs look within reason and it should run.

Is the bore cracked anywhere ??

Hope this helps,

Jim
  #6  
Old 03-19-2003, 01:15:12 AM
BobRR
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Default Re: 1-1/2 FM ZA compression

Al if you have .008 between the ring and the land you have to much.It should be closer to .002 You need to have the land cut and go to the next thicker ring or get .030 spacers from Otto. You wont believe how much tightening that clearanace up will make on compression.There are several threads on this in the archives too. BobRR
  #7  
Old 03-19-2003, 11:56:53 AM
Pistonring
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Default Re: 1-1/2 FM ZA compression

Al- .008" is way too much side clearance if you want good compression. BobRR hit it square on the nose. Vernon and Jim Tr. I beg to differ with you both. The problem will not be solved by any honing boring or sleeving. If the ring grooves are not straightened up the problem will not go away. The only thing that will go away is your money that is wasted on sleeves etc. Increased break in time may help some but once the grooves have over 5 thou of side clearance things go south fast. The only other thing that would force the rings to seat is to pin them to the piston. If you go to this trouble why not just fix it right and turn the grooves. I have had many people call with new rings, and fresh properly fitted sleeves that had still no compression. Until the ring grooves were straightened compressionville was pretty empty. Since you already have a new set of rings it is not necessary to buy more. Get a .030" spacer and put it on the top side of each ring. The tune should change then. Go to: http://www.dol.net/~dave.reed/tips.htm and check out my page that has more information on this subject. Thanks, Dave Reed Otto Gas Engine Works 2167 Blue Ball Road Elkton MD 21921-3330 phone 410-398-7340 http://www.pistonrings.net




Piston ring tips page - click here.
  #8  
Old 03-19-2003, 02:33:10 PM
Jim Tremble
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Default Re: 1-1/2 FM ZA compression

Thanks Dave

I was always lead to believe .010 was the max on ring side clearance on larger bore engines but have never seen them that loose.

I learned something today.

Really, Thanks for correcting me.

Jim
  #9  
Old 03-19-2003, 11:08:27 PM
Al Hettich
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Default Re: 1-1/2 FM ZA compression

Thanks guys! The bore is not cracked or scored. There is a good possibility that the grooves are the problem, and I am not sure that more running in will do it. I forgot to mention that as the surplus oil or grease wears or burns off, the rings make a chirping, squeeking sound. This leads me to believe that this sound which I have never heard before is generated by the rings being loose in the grooves. As the rings began to break in it did not get better. I also have engines with fair compression that have worse cylinder wear. Al
  #10  
Old 03-21-2003, 10:42:40 PM
Al Hettich
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Default Re: 1-1/2 FM ZA compression

Got my rings and spacers from OTTO/Dave Reed today. I machined the ring grooves wider to accept the spacer shims. The grooves were tapered very badly. The old/new set of rings had the black coating worn off on just a very narrow part at the bottom of the ring, but they looked to be breaking in very nicely. After fitting the spacers and rings, the engine started on the third pull. It had many times more compression than before. I believe that after the rings break in it will be very good. Thanks for the help and Dave for the super fast service. Al
 

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