Help with Puch

I have a knocking noise coming from my cylinder area of my 1977 puch maxi sport hi torque.. I am assuming this is "piston slap". I opened it up to take a look and found that the piston has a lot of side to side play where the wrist pin and the connecting rod meet in between the circlips. The connecting rod alone is not moving, Is there any way to fix this why is there more play now than before.

Thanks,

Chris

Re: Help with Puch

i dont understand.. the connecting rod isnt anywhere near the circlips.. are you saying the piston pin is able to move back and forth in the space between the clips?

piston slap noise is due to (excess) clearance between the cylinder wall and the piston, but there's some small amount of slap in any engine.

when and where is the knocking sound most evident.. top speed, low speed, starting or idling, etc.

Re: Help with Puch

at idle and up to 20 mph i can hear the knocking it is loudest at idle and under 10mph. The play is when I wiggle the piston from side to side while connected. The crankshaft rod slides back and forth quite a bit on the wrist pin, maybe half inch?. My terminology could be wrong. I did recently sand the piston a little, because it was a little rough, could this be causing the piston slap?

Re: Help with Puch

piston/bore clearance is determined by measuring the piston and cylinder bore diameters.

Clearance between the piston's widest point (near the bottom, cross-wise from the pin) and the cylinder should, ideally, be about 0.002 inch.

your terminology is a bit confusing.. the "connecting rod" connects the piston to the crankshaft.

There are 4 Puch manuals on THIS page .. one or more will no doubt include instructions on measuring clearances as well as specifications..

there are lots of reasons an engine might make unusual noises or too much of some peculiar noise... no need to settle on piston slap at this point.

Re: Help with Puch

to fix the side to side movment of the piston you need a new wrist pin bushing they are usually brass and you can get one from moped junkyard

Re: Help with Puch

wrist pin bushing? where is this bushing?

quite a bit of piston side-to-side movement is normal when the cylinder is removed from the engine .. normally you can wiggle the piston sideways just because the connecting rod big end has some clearance.

Re: Help with Puch

hmm.. now it starts to make sense ..

if a big-end bearing is toasted there will be way too much big-end clearance.. and lots of noise .. and the piston movement side to side will be excessive.

Try this.

Bolt the cylinder firmly back onto the engine with a couple of the head studs and nuts, using appropriate shims or a stacks of washers.. Leave the head off.

Turn the crankshaft so the piston rises up to and goes just a hair beyond top-dead center.

Pound the top of the piston with the heel of your hand (or a padded, totally silent hammer-type tool)

Listen for a obvious click sound.. try it a couple times.

if you hear a click, a bearing or something, somewhere in there is probably gone.

Re: Help with Puch

I havent test by pounding it yet, but the reason I dont think it is a bearing is because the the noise has come only since I installed a hi torque piston in with only the top ring installed because I broke two sets of bottom rings (next time I will use zip ties or hose clamp). anyway, the motor was running quiet until I put the new piston in.

The piston is matched to the cylinder, both hi torque.

Would the bearing sound like a clanking, because I've heard bad crank bearings and it does'nt sound like that. It is a definite tapping sound.

Chris

Re: Help with Puch

check piston-to-head clearance.

Re: Help with Puch

If my engine is a 2hp hi torque should I not be using a head gasket. Could this have anything to do with it?

Re: Help with Puch

Gasket or no gasket, something's banging into something unless a bearing is gone ..

Since this started with a piston modification it seems reasonable that the piston has a problem or it is a part of the problem, or it's installation has revealed the engine has a problem elsewhere.

Get some thin, soft resin-core electrical solder. Bolt the engine together, including whatever gaskets are supposed to be there. Remove the spark plug and insert a strand of solder so it reaches the outside edge of the combustion chamber.

Crank the piston over beyond TDC, crushing the end of the solder. Remove the solder and measure it's crushed thickness. If it's less than about 0.02 inch (or half a millimeter) thick, things are too tight. Once the engine is up to running temperature this clearance will decrease, perhaps to less than zero.

Higher torque comes from a higher compression ratio. I'm guessing this "high torque" piston is probably taller than stock.. it's crown probably approaches the head closer than a stock piston would, reducing combustion chamber volume and so increasing compression ratio.

This modification may work just fine on an engine that's in good shape where connecting rod bearing clearances are within specs. But if something a bit loose, the piston may be able to contact the head. So you may have to resort to a head gasket to increase clearance..

this is all speculation until you get down and investigate things..

Re: Help with Puch

joew,

I think it is a clearance issue. I don't quite understand the solder trick. Can I get the solder at a home depot, lowes, etc?

And do I just stick the solder in the hole, does the solder have to be at an exact depth? And what do I measure it with>>?

Also does anyone know if I can find circlips at a home depot that would fit a piston?

Thanks

Re: Help with Puch

get some thin electrical solder like at Radio Shack .. get resin-core. (It's not solid metal.. the core is a paste/flux and this solder crushes very easily.)

The piston most closely approaches the head at the outside perimeter of the combustion chamber.. the "squish band". So, that's where minimum clearance would be measured.

Bring the piston up to maybe 1/2 inch from TDC. Then stick a length of this soft solder-wire into the plug hole and probe around.. you want to feel the end of the solder rest against the cylinder wall.

Hold the solder still and turn the piston over past TDC, crushing the end of the solder. Measure it's thickness with a caliper. This one !

is $4.99 .. bottom of THIS PAGE but i recommend you get a nice one..

:

If there's a piston-to-head clearance problem, it would be cured by a (thicker) head gasket. So, a simple test would be to install a thick head gasket and run the engine.. if the ticking noise disappears you've likely found the source of the noise. The ultimate cure may not be so simple, depending on exactly why the piston is hitting the head.

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