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Big End Slop


John McClean 1
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Hi. Recently I was given a model with an Irvine 53 engine. I ran the engine and a lot of black muck came out of the exhaust to silencer joint. Looking inside I found the bearings were rusty, it had been lying around for too long. I've cleaned it up and fitted new bearings. When I turn over the engine, without the backplate or the glow plug fitted, I can see and feel the big end 'click' as it goes over TDC. Do I need to change the big end bearing, or put an extra shim under the head, or would metal expansion when it's running alleviate the problem.

Regards, John

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Sure it's not just tightness at the top of the piston/bore?  There will always be a slight clearance at the big end which would be detectable by eye in these circumstances. An easy test for any ovality would be to see if there's detectable difference at different orientations.  Black muck is usually due to aluminium fretting e.g. a loose exhaust flange.

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Hi Martin. I'll check it at different orientations tomorrow. I think the engine has had very little running time, the piston head is quite clean. The exhaust gasket burned out when I ran it, perhaps I didn't tighten the silencer enough. Have you any suggestions for a material to use for a gasket ?

Thanks

John

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within reason clicks over TDC are normal as the tolerances stack up. You need clearance around all the parts (big and little end, gudgeon pin in the piston) and all you are feeling is these 3 collapsing one way, and then expanding again the other way. Nothing to worry about

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Don't bother with a gasket on the exhaust. They always compress and come loose and if your unlucky you loose the bolts and occasionally the muffler as well. Use a drop of  epoxy on the mating exhaust surfaces making sure the bolts have plenty of oil on the thread before tightening. This epoxy will stop any leaks or black gunge . When you want to remove just undo the bolts and tap the exhaust sharply . 

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Hi. Sorry but I got waylaid for a few days. The Irvine 53 is running fine now, I used small bit of silicon gasket sealant at the exhaust flange. I'll have the model finished soon and try it in the air. On a another point, often the engine mounting set screws need to be longer than the space between the exhaust flange and the mounting lug, the head of the screw won't go past the exhaust flange. I've fitted the screws from underneath, i.e. with the nuts on top. Are there any other suggestions? I don't want to grind half the head off !

Regards

John

 

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Suggestion 1:  If you bore out the two mounting holes on the exhaust side by an extra 0.5mm then you might be able to insert your bolts at just enough of an angle for their heads to clear the flange, and once the heads are clear then the bolts can drop plumb down onto the mount, etc.  I wouldn't worry about sloppiness, as the other two holes will be a snug fit to align the engine correctly, and once all the nuts are fully nipped-up the engine won't be going anywhere.

 

Suggestion 2:  I haven't measured the difference myself, but perhaps a hex-headed bolt will have a smaller head diameter than a conventional one?  Even if this isn't the case then filing just enough of a flat on the outside to clear the flange might do the trick, without compromising the strength of the rest of the head to hold an Allen-key while you nip up the nuts on the other side.

 

Edited by Jonathan M
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