Holed piston!!

[lamscuffer]

Hello all, am running a tm35(was),when it went bang was doing 70/75 overtaking on the motorway :biggrin: .
It does concern me that there might be an underlying problem other than crap fuel that caused it to happen, as it looks like the barrell may need replating and a new piston so not gonna be a cheap fix and i dont want it happening all over again!!!!!!!!!!!!
As it is the family holiday is now cancelled

[richie]

Hi there, Super unleaded contains a higher percentage of ethanol, i'd stick to stock 95ron unleaded and add octane booster if you need to. I also use a quality oil that is compatable with alcohol fuels

Wrong, Super Unleaded contains less, and in lots of cases, Zero Ethanol. Check this out :-

http://www.groups.tr-register.co.uk/wes ... pdate.html

I stand corrected. I have to say i've yet to have a problem on 95ron with 2% octane booster added. Maybe time to up the compression and run Super!

[rbgaz]

change u piston for a super monza piston set u timing no more than 16 put a 360 main and 27 pilot jet re profile a new head to suit monza piston get u revs down change u gearing for top end if running gp box go from a 5.2 to 4.8 put a road pipe on to pull it like a franspeed road put either or both a cht and egt gauge on run a 10 plug as well i can bob a long at 65 with temp reading on cht of 160 pretty good

[Steve S2]

Had 2 scooter hole their pistons within around 2 mile of each other yesterday,
First was a ts1 and the second was a mugello 186,
We had just had a fuel confirmed was ok on a dyno, we set of in the morning to the dyno around a 40 mile trip. Both scooters run great, we then put them on the dyno and both where bang on with the jetting so no work was needed to be done.

We set of for home again a 40 mile trip. Both running great, half way home we pull into a shell garage to fuel up, around 6 ltrs of super unleaded petrol was put in. Both using different oils, after around 8 miles the ts1 pulls over, I then go back to see where he is only for my mugello to do the same,

After stripping them down last night, both had a holed piston,

The only common factor between the 2 scooter was the petrol station where we stopped to fuel up.

[mick1]

The usual cause for a holed piston (i did 2 in a month!!) is a weak mixture...too little fuel or too much air. You may have found a lean spot in your set up or a leak around the carb (rubber) would be a good guess. Check your flow rate out of your tank as this could also be a factor.

[Rich_T]

The most common dynos around are inertia dynos. These will not replicate a fixed load on an engine at a fixed speed. The most common example of this would be accelerating to 60mph then backing off the throttle to cruise at 60mph. Once the throttle is backed off in this condition the AF ratio will be different from when the dyno would read AF ratio at that speed. Consequently there has to be a very good proportion of sensible judgement in selection of mid range jetting. The principle difference being road use and application not hard acceleration.

I am strongly of the opinion that a large majority of holed pistons on alloy kits are not soley down to dodgy fuel but a combimation of over confident compression ratios and over lean jetting for road use. Just because the piston gets holed in the last 6 miles does not signify that the root cause has been in the last 6 miles. More like the preverbial straw has only been recent, be that fuel or a combination of weather conditions.

Reliance on inertia dyno to deliver you good data to build a reliable road engine is pretty risky, there is a tendancy to chase settings for the chest beating BHP reading and this leads to leaner jetting and higher compression. If this route is followed then it's only a matter of time before disaster strikes.

There is a certain amount of denial about alloy barrel reliability. 10 years or so ago, it was a common cry that plated alloy barrels are fantastic because they don't sieze like iron barrels. For sure, an alloy barrel is far more forgiving with jetting and on the dyno this is commonly over looked in the persuit of BHP readings. The peril in this approach is that it stokes up expensive problems for the future. Unlike an iron kit, the alloy barrel will not warn you with a light seize, it just holes the piston and possibly a lot worse.

If you want to reliably get to your destination then it is common sense to built a road going engine to take into account variations in pump fuel and this means not pushing compression and jetting to the limit. You have to remember these engines are late 50's, they lack all of the sophistication of a modern two stroke so you have to listen to them carefully.

[Doom Patrol]

We want to be making our engines bullet proof and not increasingly fragile.

[Adam_Winstone]

The most common dynos around are inertia dynos. These will not replicate a fixed load on an engine at a fixed speed. The most common example of this would be accelerating to 60mph then backing off the throttle to cruise at 60mph. Once the throttle is backed off in this condition the AF ratio will be different from when the dyno would read AF ratio at that speed. Consequently there has to be a very good proportion of sensible judgement in selection of mid range jetting. The principle difference being road use and application not hard acceleration.

I am strongly of the opinion that a large majority of holed pistons on alloy kits are not soley down to dodgy fuel but a combimation of over confident compression ratios and over lean jetting for road use. Just because the piston gets holed in the last 6 miles does not signify that the root cause has been in the last 6 miles. More like the preverbial straw has only been recent, be that fuel or a combination of weather conditions.

Reliance on inertia dyno to deliver you good data to build a reliable road engine is pretty risky, there is a tendancy to chase settings for the chest beating BHP reading and this leads to leaner jetting and higher compression. If this route is followed then it's only a matter of time before disaster strikes.

There is a certain amount of denial about alloy barrel reliability. 10 years or so ago, it was a common cry that plated alloy barrels are fantastic because they don't sieze like iron barrels. For sure, an alloy barrel is far more forgiving with jetting and on the dyno this is commonly over looked in the persuit of BHP readings. The peril in this approach is that it stokes up expensive problems for the future. Unlike an iron kit, the alloy barrel will not warn you with a light seize, it just holes the piston and possibly a lot worse.

If you want to reliably get to your destination then it is common sense to built a road going engine to take into account variations in pump fuel and this means not pushing compression and jetting to the limit. You have to remember these engines are late 50's, they lack all of the sophistication of a modern two stroke so you have to listen to them carefully.

Rich, well said! Another current problem that I see time and time again, which I feel is totally linked to 'dyno tuners' is that today's improved ring quality is causing the tuner to feel too confident in the piston's ability to cope with wider exhaust ports. Yes, a dyno will always highlight the performace improvement that results from wider exhaust ports and increased areas, however, just because the rings will cope for perhaps a few thousand miles doesn't mean that it is the right thing to do. Many of the people that I've known to drop rings on expensive kits have had them ported by tuners that have a dyno in house and examination of the damage does suggest that the rings had been dropping into the exhaust port before they finally gave up and broke, taking the piston and bore with them

I think that there needs to be more consideration of performance vs long-term reliability.

Adam

[soosh]

The most common dynos around are inertia dynos. These will not replicate a fixed load on an engine at a fixed speed. The most common example of this would be accelerating to 60mph then backing off the throttle to cruise at 60mph. Once the throttle is backed off in this condition the AF ratio will be different from when the dyno would read AF ratio at that speed. Consequently there has to be a very good proportion of sensible judgement in selection of mid range jetting. The principle difference being road use and application not hard acceleration.

I am strongly of the opinion that a large majority of holed pistons on alloy kits are not soley down to dodgy fuel but a combimation of over confident compression ratios and over lean jetting for road use. Just because the piston gets holed in the last 6 miles does not signify that the root cause has been in the last 6 miles. More like the preverbial straw has only been recent, be that fuel or a combination of weather conditions.

Reliance on inertia dyno to deliver you good data to build a reliable road engine is pretty risky, there is a tendancy to chase settings for the chest beating BHP reading and this leads to leaner jetting and higher compression. If this route is followed then it's only a matter of time before disaster strikes.

There is a certain amount of denial about alloy barrel reliability. 10 years or so ago, it was a common cry that plated alloy barrels are fantastic because they don't sieze like iron barrels. For sure, an alloy barrel is far more forgiving with jetting and on the dyno this is commonly over looked in the persuit of BHP readings. The peril in this approach is that it stokes up expensive problems for the future. Unlike an iron kit, the alloy barrel will not warn you with a light seize, it just holes the piston and possibly a lot worse.

If you want to reliably get to your destination then it is common sense to built a road going engine to take into account variations in pump fuel and this means not pushing compression and jetting to the limit. You have to remember these engines are late 50's, they lack all of the sophistication of a modern two stroke so you have to listen to them carefully.

Rich, well said! Another current problem that I see time and time again, which I feel is totally linked to 'dyno tuners' is that today's improved ring quality is causing the tuner to feel too confident in the piston's ability to cope with wider exhaust ports. Yes, a dyno will always highlight the performace improvement that results from wider exhaust ports and increased areas, however, just because the rings will cope for perhaps a few thousand miles doesn't mean that it is the right thing to do. Many of the people that I've known to drop rings on expensive kits have had them ported by tuners that have a dyno in house and examination of the damage does suggest that the rings had been dropping into the exhaust port before they finally gave up and broke, taking the piston and bore with them

I think that there needs to be more consideration of performance vs long-term reliability.

Adam

Now im sure ive heard this before somewhere.................................................................................................................................................

[Kieron maguire]

Did you check if your carb rubber manifold was split or carb tightens up properly , also any cracks in your pipe especially if it's stainless , mine split and trashed my piston