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Old February 15th, 2012, 10:57 AM   #1
Skippii
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Unhappy Help! Need opinions on pistons...

Huh...this is kind of a weird feeling...I'm usually the one giving advice in this section, rather than requesting it. Especially since, due to space constraints and where my motorcycle broke down, I had it towed a mechanic who's doing the work rather than doing it myself.

Left Cylinder compression test reads 170psi. Right hand reads 0psi.

Here are the pics of my engine.











Obviously the right piston's gotta go. How bad is the left piston? Mechanic recommends I replace the piston, and hone the cylinder walls, which I would do if I had any money, but since I don't, I'm thinking that good compressions means it's not critical.

Opinions on piston manufactures? OEM, aftermarket, used on ebay, etc? I'm not interested in anything that will require higher octane gas or that will cost much more.
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Old February 15th, 2012, 11:30 AM   #2
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You'll want to put an OEM piston back in so that it matches the other one. If you get another brand you should replace them in pairs. It looks like that cylinder went lean, melting a hole in the piston and causing excessive thermal expansion, hence the scuffing of the piston walls. If you replace the rings you have to hone the cylinder, otherwise the rings won't seat properly and you'll be using lots of oil.
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Old February 15th, 2012, 11:36 AM   #3
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Quote:
Originally Posted by FrugalNinja250 View Post
You'll want to put an OEM piston back in so that it matches the other one. If you get another brand you should replace them in pairs. It looks like that cylinder went lean, melting a hole in the piston and causing excessive thermal expansion, hence the scuffing of the piston walls. If you replace the rings you have to hone the cylinder, otherwise the rings won't seat properly and you'll be using lots of oil.
Okay, I should have clarified that a bit. The options are to replace and hone just the one cylinder/piston/rings with OEM, or to replace and hone both of them (possibly with aftermarket.) Which is basically exactly what you said.

Main question is, is it worth replacing the piston that still works fine and shows 170psi?
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Old February 15th, 2012, 12:21 PM   #4
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If you are paying a mechanic , I'd conssider just getting a used motor in decent shape to replace this one. SHould be able to get one for ~ $250 or so.
If I was going fix this motor, I'd work on freshening both cylinders/pistons /rings. my 2 cents worth
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Old February 15th, 2012, 01:24 PM   #5
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If you are paying a mechanic , I'd conssider just getting a used motor in decent shape to replace this one. SHould be able to get one for ~ $250 or so.
If I was going fix this motor, I'd work on freshening both cylinders/pistons /rings. my 2 cents worth
If I was doing this myself, I'd just swap engines...done that before and it's pretty simple. But since the bike is almost 200 miles from my garage, I'm letting this mechanic make most of the calls. I know him, and he knows his stuff better than any other mechanic I've met.I know it'll be more expensive, but I'll feel better knowing he built it vs a used engine. Just spoke to him again on the phone, he'll be honing both cylinders tomorrow. Then we'll know if stock pistons will fit, or if we'll need oversized pistons or new sleeves. So that will basically determine if it's even possible to reuse the left piston.
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Old February 15th, 2012, 03:12 PM   #6
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Just out of curiosity... How many miles are on that engine?
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Old February 15th, 2012, 04:05 PM   #7
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There isn't a way in the world that I would reuse that piston. Put new ones in.
It has had most all of the abnormal use that the failed one has had. It just hasn't failed yet. The potential cost of redoing all that work a few hundred or few thousand miles down the road should be your deciding factor. Not to mention the further inconvenience.

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Old February 15th, 2012, 04:22 PM   #8
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Quote:
Originally Posted by Skippii View Post
Okay, I should have clarified that a bit. The options are to replace and hone just the one cylinder/piston/rings with OEM, or to replace and hone both of them (possibly with aftermarket.) Which is basically exactly what you said.

Main question is, is it worth replacing the piston that still works fine and shows 170psi?
Pistons don't really wear out, per se. What wears is the ring/wall interface. If you replace just the one piston the main issue you might have is a little more vibration from the new bore having slightly better ring sealing. Might. The other piston, if the rings are sealing well and it's not damaged, will be just fine. What happened to the piston that failed was a problem with the carb on that cylinder, not an overall engine problem. Since the Ninjette doesn't use a shared manifold intake system problems are more limited to just the affected bore, unlike something like an airplane engine which may have a single carb feeding multiple cylinders. Also, unlike airplane engine you don't have to rebuild the engine perfectly to meet any FAA regulations.

You can if you want to, but that will get expensive quick, and IMHO isn't really necessary unless you were racing.

I'd just replace that piston, and in fact, if the rings are good and it was my engine I'd leave the original rings in, they're already match-worn to the cylinder wall. But my labor is free to me so I wouldn't have a problem with popping things back apart to replace little bits like pistons.
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Old February 15th, 2012, 05:18 PM   #9
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Personally, I would have the one cylinder honed. Replace the piston and rings and call it a day.

If the other piston is making 170 psi, it is sealing fine.
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Old February 15th, 2012, 05:23 PM   #10
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There wasn't a problem with that carb on that cylinder, there was a problem with half my coolant falling out on the interstate.
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Old February 15th, 2012, 05:52 PM   #11
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Where did the other half fall out?
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Old February 21st, 2012, 06:10 PM   #12
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Where did the other half fall out?
I don't know. When I mopped it all up into a bucket from the road so I'd be able to reuse it, I only had about half of what should have been there. Maybe it evaporated?
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Old February 21st, 2012, 06:25 PM   #13
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I thought I read pistol....


reading fail.


good luck
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Old February 21st, 2012, 06:41 PM   #14
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If you tried to 'mop it up' then about 1/4th of what was there was still in the 'mop' and I would say about 1/4th you just can't get off the ground... so I'd think you got back all you could. Or you could've been just running low on fluid
As for the pistons, replace them both, if you need any help PM me I can spot ya some cash till the Lil Switz trip in Aug.
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Old February 21st, 2012, 06:51 PM   #15
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PISTONS DO IN FACT WEAR OUT!!!! along with the cylinder. Piston can be sealing fine and be close to out of spec. They wear on the front and back of the skirt and the cylinder gets out of round.

~$300 for a set of pistons. Try vertex as they make many of the OEM pistons.
Used complete running motors

Now granted, you could probably get away with using that used piston...If you are going to do that and half ass it just buy some ebay used pistons($20) and slap it together and sell it. If it is your commuter do it right and have the good feeling it is tight and done correct and you will not have problems down the road. But complete running engine on ebay...$300-$400 plus shipping.

Now the important question is...why did you lose coolant? fix that first before anything. Water pump, hose, head gasket...
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Old February 21st, 2012, 07:49 PM   #16
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To do it right.

Take the head to a decent bike shop. Have them mic the cylinders and let you know if you need +.5 pistons or not. (probably, given what the piston skirts look like)

You can have them order the pistons and rings or order them yourself. Have the shop bore the cylinder to the +.5 spec (a good shop will not bore the cylinders until they have the new pistons in hand).

Then take all the parts home and put it back together with new base and head gaskets.

Adjust the valves

Done

You should be able to get pistons, rings, cylinder bored and gaskets for a little over $200.
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Old March 1st, 2012, 02:17 PM   #17
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Quote:
Originally Posted by flynjay View Post
To do it right.

Take the head to a decent bike shop. Have them mic the cylinders and let you know if you need +.5 pistons or not. (probably, given what the piston skirts look like)

You can have them order the pistons and rings or order them yourself. Have the shop bore the cylinder to the +.5 spec (a good shop will not bore the cylinders until they have the new pistons in hand).

Then take all the parts home and put it back together with new base and head gaskets.

Adjust the valves

Done

You should be able to get pistons, rings, cylinder bored and gaskets for a little over $200.
Close.

OEM Pistons, rings, piston pins, circlips, gaskets, etc. were a little over $265. This did not include S&H, or boring.... honing in this case.

FWIW - I am the one doing the work. Decided to replace both pistons JIC there was heat damage to the "good" one. Honed the cylinders, still fell within the service limits, so decided to go back with std. sizes.

Everything is back together but another issue has popped up we're working through.
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