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ASP .46 rebuild

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Old 02-09-2007, 05:09 AM
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cathurga
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Default ASP .46 rebuild

Hi All,

Here's the deal:

A friend of mine retired his ASP .46, and I took it under my wing to get running again. The reason for retirement was that is sounded AWFUL when running, a terrible clanging cacaphony of sound I thought this was likely to be a bearing that was past it's sell-by date.

Anyway, I disassmbled the unit, and replaced the bearings. I noted that although there was some where, it wasnt chronic, but I replaced them with some units that matched from the local bearing shop (the original parts are silly expensive, and I am in the UAE, so ordering them was not an option)
I put everything back together, and while doing so, noted that the piston and sleeve look to be in good condition and without the head on, everything turns really nicely, one expects a slight 'notching' at TDC as the piston stops and then begins it's downward run. I insert the VERY thin shim gasket and tighten up the head, only to find that the piston seems to be TOUCHING the head at TDC, and really battles to get past TDC ( I expect that this is doing NO good to the bushings in the top or bottom of the crankshaft). Bear in mind that this is all happening with no plug in, so it cannot be too much compression.

Do I add another shim to get a bit more clearance? I have other 2-strokes that have NO shims and still seem to operate well enough. Any suggestions?

Thanks
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Old 02-09-2007, 05:28 AM
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Default RE: ASP .46 rebuild

What kind of fuel are you using?Nitro percentage?
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Old 02-09-2007, 06:03 AM
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Default RE: ASP .46 rebuild

Unless you lost a shim the piston doesn't just start to hit the head. If it is truly hitting it should show it on the piston or head metal. Could be original ABC pinch with the rod bushings wore out would give you a knock or clunk at top dead center.
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Old 02-09-2007, 08:40 AM
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Default RE: ASP .46 rebuild

As far as fuel is conerned, I am not sure, what fuel this guy has used, but the engine is in fair condition and it is not misfiring or firing to early, the problem is apparent turning with the hand, not only when running..
I have found out that the ASP has two shims, so I have added another (and then a third too!) and the problem is still there. As you tighten the head down, ig starts doing it. I thought about the ABC pinch, but surely that would be apparent whn the head is off too. I am starting to think that when the head is tightened down, it pushes the sleeve down further than it should go. I see no shim under the sleeve, and seriously doubt there is one in there. Maybe the sleeve is the wrong one...? There is no marking on the head or piston to suggest knocking at all....will try some engineering blue on it.

Any chance the the piston is installed the wrong way around?

Thanks for the replies
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Old 02-09-2007, 08:58 AM
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Default RE: ASP .46 rebuild

Since you did not get the engine new there is no history , you may be on to something on the wrong liner > I just looked at my ASP it is a 15 but the liner has a flange thus holding the head higher off the cylinder casting. It may be
that an earlier type ASP just had a liner that was flush with the top (no flange) and was inserted as a replacement. Far out but possible?? martin
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Old 02-09-2007, 09:06 AM
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Default RE: ASP .46 rebuild

One thing that I and a few others have found is that a piston can be quite free of any pinch without the head on but as soon as it gets tightened the liner distorts and you feel a pinch. This doesn't happen until the screws are being tightened beyond the initial barely tight stage though. Each time I tighten a screw a little more I turn the piston over TDC to make sure it still feel the same. If it goes tight I'll back off that screw and try the next in line.

Seeing your engine still goes tight after adding extra shims it won't be the piston hitting the head so try tightening the screws in very small stages and check each time you tighten one.
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Old 02-09-2007, 09:20 AM
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Default RE: ASP .46 rebuild

Does the piston protrude from the sleeve at TDC?

CR
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Old 02-09-2007, 09:46 AM
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Default RE: ASP .46 rebuild


ORIGINAL: downunder

One thing that I and a few others have found is that a piston can be quite free of any pinch without the head on but as soon as it gets tightened the liner distorts and you feel a pinch. This doesn't happen until the screws are being tightened beyond the initial barely tight stage though. Each time I tighten a screw a little more I turn the piston over TDC to make sure it still feel the same. If it goes tight I'll back off that screw and try the next in line.

Seeing your engine still goes tight after adding extra shims it won't be the piston hitting the head so try tightening the screws in very small stages and check each time you tighten one.

------------


I'm with Downunder on this one. The final, assembled "pinch" is what matters since that is what the engine will run with in the end.


Ed Cregger
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Old 02-09-2007, 11:06 AM
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Default RE: ASP .46 rebuild

Great points on the head screw thing try first< It the problem is still there , hold the head on the block ( no screws) and rotate thru
TDCif the head is displaced up by the pistion you will know right a way but make sure it is not the liner rising in the cylinder If it still hits could be wrong liner or piston, very little chance of it but check out, if all else fails martin

the piston being turned 180 degrees has no effect on the stroke this is a constant determined by rod length and
crank pin location on the shaft , there may be a cut a way on the pistion to clear either the back plate or shaft
counterweight If it faces the shaft (the cutaway) and backwards the piston skirt will hit the counter weight

an other thought maybe a head change too if it projects too far down in the cylinder it will hit the ( piston TDC)
ASP has had some changes and up datesl
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Old 02-09-2007, 04:38 PM
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Default RE: ASP .46 rebuild

Thanks everyone for the info, very helpful indeed.

Yeah, the problem only starts when I tighten the screws down the last little bit, I had done a test to see if the head was displaced by just holding it down, and there was no movement felt, but I thought that just by hand I would not be able to compress it as tight as the screws. I am going to try a few variations on the tightening of the screws.
I took the backplate off again to see if there was any undue stress on the crankpin when the head was on and rotating the shft, but it all appears normal.

Again, thanks for all the help.
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Old 02-09-2007, 06:39 PM
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Default RE: ASP .46 rebuild

I had the same thing happen with a Tower 75....it was fine till I put the final torque
on the screws....then it would get tight. I put two shims in it, and ran it....it was fine,
still running two years later. Finally I figured it was supposed to be like that.

FBD.
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