PlanetMinis Forums banner
Cancel
Create thread New Forums

What would happen if...

Jump to Latest
1 - 20 of 27 Posts
#2 ·
As long as you match the crank and piston, the deck height won't be a problem. At that point, it's just a matter of matching piston design to combustion shape/valve layout. For awhile I was running a short rod piston and crank with a 70 head, aka a long rod unit, and it worked quite well. The piston I chose was a near perfect match for the head.
 
#3 ·
Kurlon said:
As long as you match the crank and piston, the deck height won't be a problem. At that point, it's just a matter of matching piston design to combustion shape/valve layout. For awhile I was running a short rod piston and crank with a 70 head, aka a long rod unit, and it worked quite well. The piston I chose was a near perfect match for the head.
Click to expand...




What piston were you using?
 
Discussion starter · #7 ·
Yeah. Plenty. I couldn't for the life of me convince TB to send me a new piston. they say that piston would hit the valves and it's my vault. But I say that there are no signs of piston/valve contact. Even if there was, it wouldn't have hit where it broke. As you can see, there are scratches on the piston and the head dome, but none that would indicate valve/piston contact. They are all in random directions so I will assume with 85% certainty(haha assume with certainty) that they are made by little piston bits flying around for the half second between the loud noise and me turning off the bike.



Image




Image




Image




Image




Image




Image




Image




Image




Image
 
#8 ·
That looks like a defective piston to me. I dont see anything that would have caused it to break other than a flaw in the workmanship. How long was it running before it broke? Maybe the piston was smashed in shipping at some point. And then took a big to give out? The valves were not hitting. You would be able to tell on the skirt. I have a few TB pistons where the valve reliefs are all out of whack, Went un noticed untill the bike started smoking. Upon inspection you could see exactly what was happening. When the valve makes contact with the piston it will cause it to cock in some way and you will almost always see skirt to cylinder contact. That skirt looks untouched.
 
#9 ·
I agree with Motarted. I'm not an engine expert but based on my experiance in defect and failure analysis I would say that the valves had nothing to do with the piston failure.



There are a couple of options:

- the piston had with some kind of manufacturing defect, perhaps an internal crack or air bubble in the casting itself.

- the gap between the ring and ring channel was not correct and created a significant stress on the top flange of the piston. There are very sharp radius' in there and those will cause a stress concentration.



If you look at the 4th picture from the top, it appears that the failure started on the left side (the side nearest to the piston pin hole) and then moved it's way to the right. Some really close up (and clear) pictures could confirm.



sani
 
#10 ·
#11 ·
Kurlon said:
Thats an awefully thin top ring land. When I melted my 51mm setup, it failed in a similar manor, part of the top ring land busted off and started pock marking the head. My combustion chamber was a WEE bit hotter when it let go though. : ) That piston looks very similar to the one I was using...



http://www.x386.net/TTR/media/06Rebuild/HPIM0249.JPG

http://www.x386.net/TTR/media/06Rebuild/HPIM0250.JPG



You can see where nearly half of the top ring land let go.
Click to expand...
That piston is lookin good :p
 
Discussion starter · #13 ·
I'm going to e-mail TB again and try to convince them that the piston broke because of a defect. Obviously no matter what head the piston was mated to, the piston should never have broken where it did. We will make this like a petition thread to get me a new piston. I don't like not having a piston in my motor. It doesn't run well without it.
 
#14 ·
I hate to say this, but I don't think TB has to replace the piston. You were technically using it outside it's listed application by using a different head, so you tossed any warranty, implied or otherwise out the window. I'm all for companies standing behind their products, but I think many in the mini community expect a bit much from most companies.



And your desk is severely lacking in bottlecaps and old computer smegma.
 
#18 ·
Just two quick questions for you, qwokie...



1. Is that piston from the '79 Z engine that you got from vigcs...?



2. The head in the pictures looks a lot like a stock CRF/XR 50 head, with the flat-faced valves, am I correct...?
 
1 - 20 of 27 Posts
About this Discussion
26 Replies
9 Participants
Last post:
arlindsay1992
PlanetMinis Forums
posts
2.1M
members
63K
Since
2003
PlanetMinis forum is the internet's largest mini bike community. View thousands of photos, event coverage, tips, tricks and performance mods.
Full Forum Listing
Explore Our Forums
CRF50 KLX110, DRZ110, TTR90 Chinese/Import Minis - General Discussion Z50, CT70 and JDM Monkeybikes Complete Bikes For Sale - All makes and models