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Wrecked piston, any idea why?

3K views 43 replies 11 participants last post by  humle 
#1 ·
Hi guys,

On yesterdays trackday my bike suddenly lacked a cylinder and I crawled into the pit smoking a lot.. sigh..

Took the front cylinder apart today and found a hole in the piston top, but not the usual circular one that indicates overheating.

I'm running with high comp JE-pistons and have done several seasons on the bike.

Anyone have good ideas as to what caused this?

Big picture here!!!
 
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#2 ·
I don't know what caused it, but it sure doesn't look burnt. Rather looks like it fractured and a chunk fell out:O
 
#3 ·
Yeah, and that chunk is gone.. i.e. in my crankcase somewhere.. Dammit..
 
#5 ·
:dunno

The cracks are interesting, makes it look like it's been smacked with something.

What' the big end look like on that one, and the exhaust valve too?

I've seen detonation put pit marks in a piston top, but not a pit mark that large. Pre-ignition :dunno That'll hammer the crap out of a piston top and big end but it'll be a uniform force not localised like that I would have thought.

A manufacturing defect combined with a kiss from a valve?
 
#10 ·
What' the big end look like on that one, and the exhaust valve too?
"the big end" ..??

Valves looks fine, apart from a little too much deposit on one of the intake valves. I'll take a picture of the valves after work today.
 
#7 ·
interesting. there is no evidence of impact. there is no evidence of detonation. my guess is it started at the pin boss and cracked its way up and over the top.

turn it upside down and look to see where the end of the crack is.

Cracking around the boss would be the product overrevving or a bad piston or a combination of both.

pull the other piston and bead blast it clean and dye-check it for cracks. If one is gone the other will most probably be going too and as it hasn't yet failed it may tell you where the problem started.

The crown of the piston seems marginal for thickness, but my best bet is it started around the piston pin.
 
#11 ·
interesting. there is no evidence of impact. there is no evidence of detonation. my guess is it started at the pin boss and cracked its way up and over the top.

turn it upside down and look to see where the end of the crack is.

Cracking around the boss would be the product overrevving or a bad piston or a combination of both.

pull the other piston and bead blast it clean and dye-check it for cracks. If one is gone the other will most probably be going too and as it hasn't yet failed it may tell you where the problem started.
Thanks Stu... I really feel we are lucky that you joined us here at the Zone..!!

I'll take a picture of the piston from the other side after work today, then I'll post it as well.

My plan is/was to get a new piston as soon as possible and put that in to get the rest of the season without having to pull out the engine from the frame. Then in winter I can exchange the other piston as well.
Does that sound like a stupid idea or ok..?

The crown of the piston seems marginal for thickness, but my best bet is it started around the piston pin.
BTW if the pistons have done several seasons it could just be fatigue, constant high revs being the cause.
When I bought the bike from Texas 5 years ago it came with these pistons, so they have indeed taken some abuse over the years. The bike used to be a racebike for a small Texas-based team.

pistons flex around the pin and this causes cyclic failure like bending a piece of metal back and forth till it breaks. you see this in drag engines all the time but these guys are checkng every meeting.
cracks will usually start where the pin boss joins the underside of the crown in the corner where all the shapes meet. careful radiusing of this area helps but is no answer for manufacture which leaves harsh angles meeting each other.
Is it possible to do this radiusing myself..?
 
#8 ·
BTW if the pistons have done several seasons it could just be fatigue, constant high revs being the cause.

but what I said before about the start of the crack.

pistons flex around the pin and this causes cyclic failure like bending a piece of metal back and forth till it breaks. you see this in drag engines all the time but these guys are checkng every meeting.
cracks will usually start where the pin boss joins the underside of the crown in the corner where all the shapes meet. careful radiusing of this area helps but is no answer for manufacture which leaves harsh angles meeting each other.
 
#13 ·
BTW if the pistons have done several seasons it could just be fatigue
:stupid

I'm with Stu, no evedence of detonation or valve contact. The 98mm Piston is a big lump and fatigue will creep in over the years.

You have been very lucky it broke the way it did without self destructing the engine.

The odds are that you would get away without having to replace the rear piston until the season end but you've got to ask yourself if a days labour and a few gaskets is less hassle than a new engine if the rear piston lets go and throws a rod through the cases and transmission, not to mention if that happens at high RPM on a fast bend on track :O
 
#12 ·
I have some new j&e's here I will have a look under later and get back to you.
radius yoursself probaly yes
 
#14 ·
That sounds REALLY good!! Thanks a lot.

With regards to radiusing myself, I have access to a full blown workshop.. Including CNC-milling and stuff.. Though I guess it's a matter of doing some old fashioned work with my hands :laugh
 
#18 ·
In a recent issue of German 'Motorrad' magazin they had a 1098 DUC with ~ 25.000kms on the clock. During testrides on the Nurburgring the bike suddenly lost power and smoked. After opening the engine they found a piston that looks pretty much the same as yours (crack and a small piece missing). They put it on fatigue due to high revs. According to the mag, rumors are this problem occured more than once during development of the 1098 enginge.

JoE.
 
#21 ·
oh crap! I keep writing things the forgetting to hit the post button!!!!!:banghead

so I got out a new J&E (thanks again sam) and the crack you have seems to start where the piston pin boss meets the underside of the piston in the inlet cutouts.
It runs between the two bosses across the crown . the bit that has come out looks like another crack has tracked from the boss to the underside of the bit that sticks out between the valve cutouts, being a cast piston there is a corresponding hollow on the underside. so you have a crude sort of triangle of cracking following stress risers under the crown.

I have some 100mm pistons and the underside seems pretty well finished radius wise. outside the pin boss there is some ribbing under the ring lands that is a bit sharp but I doubt your cracks started out here.On the whole i'd say the crown is a bit thin and may not offer enough rigidity to the pin bosses. they are a lovely piston and definitely light for what they are. there is always a compromise in the construction of components. strong enough to survive , light enough to work at high revs and not break the things hold on to them. I'd say this is a good lesson in the longevity of these units. after every season pull down and crack check and after several pull and test every couple of meetings.
When you have the chance dye check the rods and crank pin radius area and at the end of the season get them magnafluxed by an engine reconditioner or better still by a truck crankshaft regrinder. they should have a tester capable of seeing cracks up to 4mm below the surface of the metal.
When you get them back make sure there is no residual magnetism left in the components.
this process is very suitable for checking gear box parts too

now cross fingers press post button
 
#22 ·
:)Sorry mate not parting with these :devious

but if it is on std bore just drop a std set of pistons in till the end of the season. tho the cutouts in your J&E look like you may have oversize valves fitted.

Mate if I was there I'd weld that old one up for you. I've done it myself on a saturday night and run again on sunday . its uncomfortable racing with your fingers crossed but a necessary discomfort somedays ::)

good luck getting it back to the track
 
#24 ·
Mate if I was there I'd weld that old one up for you. I've done it myself on a saturday night and run again on sunday .
:laugh We're not talking aircooled RD's now Stu when araldite fixed holes in everything :laugh
 
#23 ·
Try TTS Performance in the UK, thats where I get my JE Pistons from :thumbup

TL Pistons aren't listed on their website but they still do them in 98 or 100mm etc
 
#26 ·
Yeah, that's a possibility.. Right now I'm considering wether to call the season at end... and get a pair of cylinders prepped for that extra set of +3mm pistons I have here in front of me :devious :devious
 
#27 ·
We're not talking aircooled RD's now Stu when araldite fixed holes in everything
damn it sam are you psychic or something? :banghead :devious :devious


 
#28 ·
damn it sam are you psychic or something? :banghead :devious :devious
Maybe, just come from a similar era and background when funds were tight and it was a case of go home or try to repair it in the pits:lol

You could just about get expoxy to harden enough to get you onto the grid before the next race if you managed to push it back in time :laugh

Back at my parents house I have a shelf above the fireplace full of Piston Trophy's with various holes/stages of meltdown from my 2-Stroke Days......
 
#30 ·
It was widely known that I had a tig and a pretty fair workshop. At the end of saturday there would be a small line of blokes with broken bits hoping to get them repaired. I'd take them back to the shop and weld, bend, straighten whatever and be just about the most popular bloke on sunday morning.
 
#31 ·
#32 ·
Pictures from rearside of piston

Hi Guys,

Here are two pictures of the piston rear side.

You can see there is quite a few cracks on the rear side as well, both close to the pin and beneath the missing bit in the middle.

Btw: Click pictures for big ones :devious



 
#33 ·
As a side note, it looks as though there's a fair bit of metal that could be removed without loosing any strength - perhaps rounding off the lower part of where the pin goes:O Might be hard to do by hand and still keep the piston balanced properly (side to side, wouldn't want it to rock one way more strongly than the other), but perhaps not impossible:deal
 
#34 ·
I would get JE to look at those pics :O
 
#37 ·
I can of course send them a mail with links to the pics. I doubt however that they will look much at it, but it's worth a shot.
 
#36 ·
I've seen a pistion posted before that failed in an almost identical manner as that one. Now I'm not sure which forum it was:confused Might have been a KLR on advrider:confused
 
#38 ·
They appear to have a satisfactory radius. only thickening the crown would give you more support and the ability to increase radius. the piston is not particularly strongly designed in the boss structure. It is very plain ,the +2's I have are quite different. tho maybe no stronger.I had wisecos in my 900 duke years ago and they were 500thou thick in the crown between the bosses. ridiculous I took about 20grams of metal out of them these are about 100thou or less . real good for weight . its all a compromise . make them heavy in the right place they get stronger, but the more strain they put on rods etc. got to find the balance. I'd say these guys did a stirling job. they just ran a season too long.

Btw these cracks would have developed over time and regular stripdown and inspection with dye check would have picked it up in good and safe time.

thanks for the pics. I really love this process of playing detective!!!
 
#42 ·
And thanks a lot for all the time spent on looking at my pics and the wise words you write.

One thing is for sure.. My +3mm JE's are lighter than the stock TLR pistons. Thus these stock bore high. comp. pistons are probably even lighter.
 
#39 ·
When looking at the pics, I just can say WHOW, that's what I call "just in time inspection".
Even when I don't think it has a relation: How does the spark plug look??

Greetings
Rufer
 
#41 ·
When looking at the pics, I just can say WHOW, that's what I call "just in time inspection".
Even when I don't think it has a relation: How does the spark plug look??
You mean, apart from the plug being drowned in oil? :laugh:laugh:laugh

Didn't look much at the spark plug, but will do when I get back in the workshop.
 
#40 ·
I think J&E would be interested . most manufacturers and designers like to have feedback. especially if it helps them improve their product. I doubt they would admit any neglect in design and I personally think these pistons have performed admirably, but they may learn something useful from it.
 
#43 ·
funny my garage door springs always break within a few days of each other. wrt to break in, i would be interested in sams comments. i did something similar to your suggestion but spent the time either WOT or closed throttle, no cruising. i guess thats easy to do on the track?
 
#44 ·
I'd say that depends on the track. Two of the tracks very near to me are rather short, curvy and with plenty of hills. They would be great for this sort of "exercise" :laugh

I get your point with the garage door spring, but still think the two pistons lives live that are different enough as to not break within the same few hours.
It's no problem now though, since I can't get a new piston in time.. Thus I'll ride my mates R6 instead and exchange both pistons during the winter.

Greetings
Johnny
 
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