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Engine piston wont cycle

Old 08-06-2009, 01:53 PM
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blhollo2
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Default Engine piston wont cycle

hey all, i need your help, i just replaced the connecting rod on my super tigre 75 engine. I put on the cylinder head on the engine on and bolted it down and tried to turn the engine over but couldnt, the pistion was hitting the top on the cylinder head when the shaft was turned..what is the problem here and how do i fix it? i've taken 2 engines apart and never had this happen [ul][*] [*] [*]
[/ul]
Old 08-06-2009, 01:57 PM
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GaryHarris
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Default RE: Engine piston wont cycle

Could eithet be you left out the head shim, or the conrod is hitting the case.
Old 08-06-2009, 01:59 PM
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Default RE: Engine piston wont cycle

Are you sure it was actually hitting or is it just really good compression? If you take the head off and turn the crank, does the piston come out the top of the shaft? If it does, then it is indeed hitting the head. If it is, then I have no idea. My suggestion would be to double check that you have everything in the right position and maybe pull it out and put it back in just to be sure you did it right.
Old 08-06-2009, 02:04 PM
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blhollo2
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Default RE: Engine piston wont cycle

yeah its definatly making contact with the cylinder head, i may have to disassmble and try it again, the head shim was in place too
Old 08-06-2009, 02:11 PM
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GaryHarris
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Default RE: Engine piston wont cycle


ORIGINAL: blhollo2

yeah its definatly making contact with the cylinder head, i may have to disassmble and try it again, the head shim was in place too

Do you still have the old conrod? If so, check the length between the two. Maybe they sent you the wrong one.
Old 08-06-2009, 02:11 PM
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Default RE: Engine piston wont cycle

Is there a raised spot (dome or ridge) on the piston that is hitting the head? If so the piston OR the head could be mis-aligned.
Second question - does the eng fully rotate with the head off?
Just noticed part about new connecting rod. Most likely wrong length.
Old 08-06-2009, 02:19 PM
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blhollo2
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Default RE: Engine piston wont cycle

thanks! it does fully rotate with the head off, the connection is the spec rod for this engine, i think it might be misaligned but the engine lacks of any way to know if it not
Old 08-06-2009, 02:21 PM
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Default RE: Engine piston wont cycle

Is the piston top completely flat? or other?
Old 08-06-2009, 02:23 PM
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blhollo2
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Default RE: Engine piston wont cycle

Flat
Old 08-06-2009, 02:28 PM
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Default RE: Engine piston wont cycle

OK, so you have a G-75 as shown on pg `14 of http://manuals.hobbico.com/sup/supz0099-manual.pdf ? And you have the gasket shown as item 22 in the drawing on pg 14?
Old 08-06-2009, 02:31 PM
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Default RE: Engine piston wont cycle

If your answer is YES, I'm going to recommend that you go to the RCU SuperTiger forum for dedicated advice. http://www.rcuniverse.com/forum/forumid_163/tt.htm
Old 08-06-2009, 02:44 PM
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Default RE: Engine piston wont cycle

my bet is the backplate is improperly installed.
Old 08-06-2009, 02:51 PM
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Default RE: Engine piston wont cycle

If it goes around when the head is off, how could the back plate have ANYTHING to do with it????
Old 08-06-2009, 02:54 PM
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Default RE: Engine piston wont cycle


ORIGINAL: blhollo2

Flat
possible problems:
Con-rod is wrong length.
glow plug is fouling the cylinder head. Test by removing plug and trying to turn over.
Piston gudgeon pin is fouling the transfer ports; cylinder liner is not aligned correctly, gudgeon pin is not secured properly.
Con rod is not symmetrical. It has a front and a back. The front has a rounded machining of the big end bearing web to clear the crank pin support. The back side does not have it and fouls the support preventing the crank from rotating fully if assembled the wrong way round.
Piston ring upside down and is fouling the lip in the top of the cylinder liner that comes with wear.
Debris in the intake venturi fouling the crankshaft intake slot.
Debris in the bearings.
A fold in the space-time continuum that manifests as an inability to rotate Super Tigre cranks. (OK maybe not, just been watching Stargate SG-1)
Old 08-06-2009, 03:06 PM
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Default RE: Engine piston wont cycle

ORIGINAL: psuguru


ORIGINAL: blhollo2

Flat
possible problems:
Con-rod is wrong length.Possible
glow plug is fouling the cylinder head. Test by removing plug and trying to turn over.Possible but not likely.
Piston gudgeon pin is fouling the transfer ports; cylinder liner is not aligned correctly, gudgeon pin is not secured properly.What the heck is a "gudgeon"? If what I think - not likely.
Con rod is not symmetrical. It has a front and a back. The front has a rounded machining of the big end bearing web to clear the crank pin support. The back side does not have it and fouls the support preventing the crank from rotating fully if assembled the wrong way round.Disproved by the fact that it DOES go around without the head.
Piston ring upside down and is fouling the lip in the top of the cylinder liner that comes with wear.Did not remove/change ring. OK before-OK now.
Debris in the intake venturi fouling the crankshaft intake slot.Not relevent, it goes around without head.
Debris in the bearings.Nope, not this either.
A fold in the space-time continuum that manifests as an inability to rotate Super Tigre cranks. (OK maybe not, just been watching Stargate SG-1) Most likely so far.
OK - Gudgeon pin = wrist pin. Had to look that one up. Still not it.
Old 08-06-2009, 04:26 PM
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Default RE: Engine piston wont cycle


ORIGINAL: psuguru



A fold in the space-time continuum that manifests as an inability to rotate Super Tigre cranks. (OK maybe not, just been watching Stargate SG-1)
thats funny, so have i.
seems like a stupid question but did you switch to a different plug?
Old 08-06-2009, 04:51 PM
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Default RE: Engine piston wont cycle


ORIGINAL: moron88


ORIGINAL: psuguru



A fold in the space-time continuum that manifests as an inability to rotate Super Tigre cranks. (OK maybe not, just been watching Stargate SG-1)
thats funny, so have i.
seems like a stupid question but did you switch to a different plug?
WMU, my old school. Wrong plug? That could be if he changed it.
Old 08-06-2009, 04:57 PM
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Default RE: Engine piston wont cycle

bruce, what year did you graduate?
Old 08-06-2009, 07:59 PM
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Default RE: Engine piston wont cycle

ST like to make their engines with the piston coming within around .010" from the top of the liner and the head sitting flat on the liner. Put a straight edge on top of the liner and turn the piston to TDC and see if it lifts the straight edge. There should of course be a very small gap between the piston and straight edge. If the piston lifts it off the liner then GaryHarris nailed it with the wrong rod. There's no way a glow plug can extend far enough to foul the piston in an engine of that size. If the rod is the wrong length then the top of the piston will also sit slightly higher than the bottom of the exhaust port with the piston at BDC.
Old 08-06-2009, 11:23 PM
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Default RE: Engine piston wont cycle

ORIGINAL: moron88

bruce, what year did you graduate?
1971, BS EET
Old 08-07-2009, 12:01 AM
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Default RE: Engine piston wont cycle

If the only thing you changed was the conrod, look there.

I live by the "if it's screwed up now, what did I touch last?" principle.

Jack
Old 08-07-2009, 11:45 AM
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Default RE: Engine piston wont cycle


ORIGINAL: bruce88123

[What the heck is a "gudgeon"?
In the UK we all are throwbacks to Agincourt or Trafalgar when we were (quite rightly IMHO) "duffing up" the Frogs.
If I remember my "Cannoneering 101", a gudgeon is a twisty bit in a cannon.

Old 08-07-2009, 01:39 PM
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Default RE: Engine piston wont cycle


ORIGINAL: bruce88123

If it goes around when the head is off, how could the back plate have ANYTHING to do with it????
my bad, didn't see that post

I had a smaller super tiger (i think it was a car engine actually) lock up because the backplate was on upside down when i put it back together. It would turn it and it acted like it was hitting the head. I actually thought it was hitting the head.

Cameron
Old 08-07-2009, 02:29 PM
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Default RE: Engine piston wont cycle


ORIGINAL: Cambo


ORIGINAL: bruce88123

If it goes around when the head is off, how could the back plate have ANYTHING to do with it????
my bad, didn't see that post

I had a smaller super tiger (i think it was a car engine actually) lock up because the backplate was on upside down when i put it back together. It would turn it and it acted like it was hitting the head. I actually thought it was hitting the head.

Cameron
I've seen similar. Sorry I bit so hard on ya.
Old 08-07-2009, 04:15 PM
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GaryHarris
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Default RE: Engine piston wont cycle

Is it fixed yet?

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