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Old 07-02-2015, 11:29 AM   #1
adventurenewb OP
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V Twin piston broke. help?

I know you're thinking "wtf is he writing about a V Twin motor for on this forum?" Good question. I've had little (meaning No) results with a Road Star forum and my experience with this forum is most of you own(ed) other bikes or just have that general knowledge I'm searching for.

The bike (project): 2002 Yamaha Road Star XV1600. Air cooled, push rod v-twin. dry sump. 65,000kms.(40,000 miles)

The story: PO riding to work (commuting, not highway speeds), hears a crack/clunk, bike loses power. Tows it to his jobsite, leaves it for a bit, and it starts up later. He didn't drive it again, but it did start.
PO and mechanic friend tear into it and finds a broken piston skirt on rear piston. He sells the bike to me. I then tear down more as I want to get into the cases to flush out any debris and check and clean the oil screen in the pick up/oil pump. So I take off the front cylinder assembly and find the front piston was cracked and maybe on its way as well. So now I'm at the "Why?" stage. What would cause this to happen? Options are:

1) Lugging engine because in too high a gear (common with the cruiser crowd)
2) Wrong weight of oil
3) Poor oil distribution to the cylinder/head etc.
4) Over revving engine

Any others? Seems odd that its both pistons. Or is that odd? Maybe because its both pistons, a fault can be narrowed down as one broken piston could be fluke (metal fatigue, poor manufacture of piston).

My research tells me this is a good design from Yamaha and these bikes are capable of many many more miles.

I know that's not much to go on and I don't know how the PO rode the bike. My gut tells me he falls into the 'mostly puttered around town' with occasional highway and not excessive speeds. He owned it for 4 trouble free years and bought it used at a Yamaha dealership and it original owner was a cert. mech. at that dealership. So I'm assuming a lot here but I think it was taken care of.

Anyway, any thoughts would be appreciated. Thanks.
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Old 07-02-2015, 01:39 PM   #2
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Old 07-02-2015, 01:40 PM   #3
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Stock or aftermarket piston?

If I recall correctly, there were some issues with Barron/Wisco aftermarket pistons that had some piston skirts break back on the day
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Old 07-02-2015, 06:09 PM   #4
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Piston looks like the fuel curve was wrong. lean in one area of the throttle and rich in another area. bet is that it was rich at idle and lean at speed. run that highway speed too long it got hot spots and popped. Was it rejetted after the somewhat straight-pipes were added? (speculation about the pipes.)
Second piston could have cracked from the hard stop when the first one popped though.

pookiebear screwed with this post 07-02-2015 at 06:14 PM
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Old 07-02-2015, 06:24 PM   #5
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Hi adventurenewb,
Photo is pretty bad... Are there signs of scuffing on the skirt, what do the bores look like, what do the bottom of the pistons look like, did the crack propagate over time (look for shiny spots on the fracture line where the pieces rubbed before falling off)?
My bet is the bike was ran hard with a cold engine repeatedly or the original owner flogged it... and cracked the skirts early on, it just took this long for the skirts to fall off

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Nemosengineer screwed with this post 07-02-2015 at 06:36 PM Reason: add stuff...
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Old 07-02-2015, 06:33 PM   #6
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That is odd.
Can happen if a piston pin seizes or the piston gets so hot it becomes a press fit in the cylinder.
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Old 07-02-2015, 07:06 PM   #7
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I'm not mechanic, but Yamaha bikes are pretty stout... you can run them pretty hard and not have any issues.

However, being a rather large V-Twin that's air cooled... I'd suspect it was overheated at some point causing the initial damage which propagated over time until it finally gave-way?????????

Maybe stuck in a traffic jam... or burn out at the rally... etc. etc.

Take that with a grain of salt... as I said, I'm no mechanic.

** Oh... and I DID stay at a Holiday Inn Express recently...
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Old Yesterday, 01:40 AM   #8
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throw in the possibility of low octane fuel leading to detonation. usually that cracks the ring lands or blows holes in the piston tops but who knows what else the excessive force of detonation can do
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Old Yesterday, 07:52 AM   #9
adventurenewb OP
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Quote:
Originally Posted by flstffxe View Post
Stock or aftermarket piston?

If I recall correctly, there were some issues with Barron/Wisco aftermarket pistons that had some piston skirts break back on the day
You are correct flstffxe. The Baron/Wiseco pistons had some issues. These are OEM originals.
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Old Yesterday, 07:53 AM   #10
adventurenewb OP
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Quote:
Originally Posted by pookiebear View Post
Piston looks like the fuel curve was wrong. lean in one area of the throttle and rich in another area. bet is that it was rich at idle and lean at speed. run that highway speed too long it got hot spots and popped. Was it rejetted after the somewhat straight-pipes were added? (speculation about the pipes.)
Second piston could have cracked from the hard stop when the first one popped though.
Pookiebear, read on as I think you're on to something.
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Old Yesterday, 08:00 AM   #11
adventurenewb OP
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My apologies to all who commented thus far.

It wasn't until Pookiebear and others mentioned 'lean' and 'straight pipes' (as it relates to exhaust) I completely forgot the fact the PO did put on a Kuryakyn Hi Flow air cleaner/filter assembly. (In my defense, I got the project in pieces and items like air cleaner assembly, carb, entire rear jug, and factory exhaust pipes were removed and in a box.) No idea how long he rode it with the air system and if he ever rejetted the carb. I can call him to ask. Another piece to the puzzle.
The exhaust (not shown in the pic) is factory and not punched out. If he did not rejet that would cause the lean mixture, correct?
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Old Yesterday, 09:11 PM   #12
pookiebear
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possible at higher speed with the easier flowing filter.
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Old Today, 06:17 AM   #13
kraven
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Totally possible.

I'd also say that the skirts could be damaged also by running it hard while cold.

So, it could be the aggregate of all the issues developed by the accessories and riding habits.

Overall, you're going to need to have the bored honed or bored and honed oversize and get new pistons. See if the cylinders clean up or are scuffed too deeply to clean up.

Shouldn't be a problem once you get new pistons and put it back together properly with the right run-in and tune.
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