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Print Page oldski-doosleds' Vintage Ski-Doo Message Board
    oldski-doosleds Message Board => Help Desk (Only) => Message started by: gto4life on January 11, 2011, 10:55:55 AM
	

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Title: Assembly of 775 Engine Issues
Post by: gto4life on January 11, 2011, 10:55:55 AM

During assembly of this 775 engine I noticed during manual rotation of engine that one spot in the rotation has resistance. The bottom end was completely gone thru with all new parts. Crank rotated nicely by itself. pistons are installed and cylinders dropped down over them. Heads are snugged down but not torqd. Its looks like the pto piston is near tdc. So mag is near bdc. Could the mag piston be hitting the cyl base gasket? By hand the engine will continue to rotate as it should but you can feel that "spot" of resistance in there. Any thoughts or suggestions at this point? I'm too far into this project to disregard it. Thanks guys.


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Title: Re:Assembly of 775 Engine Issues
Post by: hermit on January 11, 2011, 11:25:20 AM

Is a ring catching the cylinder maybe due to the ports not being chamfered?


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Title: Re:Assembly of 775 Engine Issues
Post by: 1970TNT on January 11, 2011, 11:40:52 AM

EITHER RING IS HANGING ON A PORT OR HEAD OR BASE GASKET IS STOPPING THE PISTON


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Title: Re:Assembly of 775 Engine Issues
Post by: moxymech1 on January 11, 2011, 11:43:19 AM

do you have your flywheel on?

it could be rubbing on the magnets...


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Title: Re:Assembly of 775 Engine Issues
Post by: gto4life on January 11, 2011, 12:22:35 PM

Thanks for the response.
The flywheel is not on yet. I was in the process of installing when I noticed the problem. It could be a ring but it doesnt feeling like a metal on metal interference. Thats why I was suspect of the base gasket I can rotate by hand thru the resistance. I hate to tear it back down again, but cant feel good about it as it is.


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Title: Re:Assembly of 775 Engine Issues
Post by: 1970TNT on January 11, 2011, 04:18:02 PM

I WOULD TAKE ONE CYLINDER OFF AND TRY IT WITH JUST THE PTO. IF NO PROBLEM TAKE PTO OFF ANDTRY WITH JUST THE MAG CYLINDER.


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Title: Re:Assembly of 775 Engine Issues
Post by: 70Oly399 on January 11, 2011, 05:58:12 PM

maybe before you pull the top end off run the piston up to the top and look in the intake port. You should be able to see a little and can check for a gasket hanging out.


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Title: Re:Assembly of 775 Engine Issues
Post by: gto4life on January 11, 2011, 06:03:01 PM

I sent the bottom end out to be professionally done. I sent the crank and lower cases. That is how I got it back. As mentioned earlier, it rotated nicely with no resistance. I picked up the resistance after the cylinders were installed. I feel like I'm answering my own question here. Pull the cylinders one at a time to find the culprit. Its gotta be the only way to do it. It just sucks to go backwards. Thanks


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Title: Re:Assembly of 775 Engine Issues
Post by: gto4life on January 11, 2011, 06:05:59 PM

forgot to mention in previous post that when I got the crank back it was already installed in the cases. It was all set and all I had to do was install the cylinders and go from there.


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Title: Re:Assembly of 775 Engine Issues
Post by: lowtekrednek on January 11, 2011, 06:17:38 PM

as a side note:

I check every crankshaft that I install for run out.
90% of the time most of the runout is on the pto end of shaft.

if you have binding after assembly, it is still accessible to check the pto end.

the crankcase will need to be disassembled to correct any run out that is found


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Title: Re:Assembly of 775 Engine Issues
Post by: doolittle on January 12, 2011, 05:09:08 AM

GTO, the sleeve that hangs below the aluminum portion of the cylinder is very easily damaged on the 775's. Many have been bent or broken when being pried loose during disassembly, and many have been dropped. Did you slide the piston all the way through the bore before assembly without the rings to check for interference at the bottom of the stroke?


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Title: Re:Assembly of 775 Engine Issues
Post by: gto4life on January 12, 2011, 01:32:01 PM

Doolittle I did not test fit the cylinders and rotate the crank. I guess that was a missed step. Anyhow, in the next day or two I will systematically start to disassemble the engine to find the culprit.... I will post the outcome of my dilema. Thanks again.


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Title: Re:Assembly of 775 Engine Issues
Post by: 640guy on January 20, 2011, 08:03:03 PM

One other thing I ran into before, check to see if one had or cylinder has more milling than the other...maybe a piston is getting too close?...I milled my 340 one lunger too much and with hi dome piston it hit top of head but that was w/o head gasket--with gasket it became only the dimension of head gasket away from head...otherwise it will has to be ring related to cylinder..

One other thing I thought of--if any carbon in ring lands--or nicked ring lands in that area--it will cause piston to move rougher and feel hung up...


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Title: Re:Assembly of 775 Engine Issues
Post by: gto4life on January 22, 2011, 06:08:26 AM

Problem Found. Reply #2 was right on the money. Hats off to 70 TNT. I made a bonehead mistake of installing 640 head gaskets. The inside diameter of the 640 head gasket is not large enough for clearance of a 775 piston. The problem has been corrected and today I will set the timing on this engine.
Thank you gentlemen for all of your recommendations of things to check. People like you make this a great web site to hang out on and learn from others. Thanks again, and now I have more work to Doo.


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Title: Re:Assembly of 775 Engine Issues
Post by: hermit on January 22, 2011, 08:05:29 PM

GREAT! Glad to hear it was that simple and Joe's offering was it. I have a very similar situation and haven't ever looked into it. It would be nice if it was that simple.



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