Burton 83.5mm accalite pistons

[Scotty77]

https://www.burtonpower.com/accralit...b120-83-5.html Hi has anyone had a issue with the above piston, I have used the twice now and both times piston number 3 followed by 1 and then 2 melted and 4 fine. Block bored and sleeved 244 Kent cam Accuspark distrubutor Twin 45 dcoe webers (with airbox) Carter fuel pump Tuned on dyno running 32 advanced timing First rebuild only lasted a day Second rebuild last 3 race meets and on the 4th number 3 piston distorted. Running standard fuel tank would fuel surge cause this problem. Could it be the air box? We are out of ideas what is causing the issue being that the same piston go in the same order. Is it the design of the piston with the thin lip melting. Any help would be greatly appreciated

[Scotty77]

First Rebuild

[Graham]

the piston itself wont be the issue! other than to say that sort of failure is common to any x/flow which is run over advanced or runs lean at some point, the design means it pics up heat very easily. maybe fuel surge is an issue, air leaks at manifold of carb mounts may cause it. are the carbs bolted up tight?, if so the fuel could be foaming up in the float bowls causing lean conditions when racing.

what compression ratio are you running? unless you have modified the pistons to lower the CR you probably have too much

[Scotty77]

Second Rebuild

[Scotty77]

Haven't modified the piston in anyway, we didn't get to check compression but Burtons website said aprox. 12.0.1

[Graham]

you put a pic up whilst i was replying, where the cutouts taper away that leave a thin portion of piston, ideal for picking up heat, i would run a milling cutter through the valve cutout to remove that thin section of piston and reduce the heat soak and probable excessive CR a bit

[Scotty77]

Also I have just remembered the first rebuild we ran Twin 40dcoe webers then moved up to the 45's so to different sets of carbies but the same issue and the same pistons affected

Just a thought, would you think the little thin edge on the pistons could be getting hot making the cylinder fire before the plug?

[Graham]

Haven't modified the piston in anyway, we didn't get to check compression but Burtons website said aprox. 12.0.1

That is a BIG mistake! unless your running rocket fuel of some sort 12.1 is too high for a 244 cam, it dont need anything like that much compression. secondly you went off advertised compression, its an estimate, tolerances could mean you only have say 11.25 but equally you might have over 13:1

[Graham]

Also I have just remembered the first rebuild we ran Twin 40dcoe webers then moved up to the 45's so to different sets of carbies but the same issue and the same pistons affected

Just a thought, would you think the little thin edge on the pistons could be getting hot making the cylinder fire before the plug?

thin edge wont help, i would remove it, but as i said you probably have TOO much compression for the cam, it really doesnt need anything like 12.1 and you may actually way way more

[katana]

Happens often with 4V headed bike pistons - the taper gets too hot and melts and smears the liners. X-flows not my strong point but usual causes regular gas + 12:1 CR - No, No! 12:1 CR + 32 deg timing No, No! Fuel surge / lean run + 12:1 CR = dead pistons! You may get away with use of 120+ octane fuel with no mechanical changes but even then it'd be boarderline!

[Scotty77]

So you think the cam acompressions is a issue. What cam would you suggest as this comes from Burton's website and it recommends the 244

A 244 cam and stage 3 head results in 135-145bhp, although, these figures are best achieved with a recommended maximum 83.5mm bore and forged Accralite pistons, giving 1700cc. There is a cheaper option in that the compression can be raised using modified 1300 pistons in the 1600 engine, giving a ratio of around 10.3:1. Capacity is easily increased with cast pistons available up to +0.090" oversize which will give 1696cc

[Graham]

So you think the cam acompressions is a issue. What cam would you suggest as this comes from Burton's website and it recommends the 244

A 244 cam and stage 3 head results in 135-145bhp, although, these figures are best achieved with a recommended maximum 83.5mm bore and forged Accralite pistons, giving 1700cc. There is a cheaper option in that the compression can be raised using modified 1300 pistons in the 1600 engine, giving a ratio of around 10.3:1. Capacity is easily increased with cast pistons available up to +0.090" oversize which will give 1696cc

i wouldn't recommend a change of cam, i would recommend, modifying the pistons and lowering the compression to a sensible figure!

experienced engine builders would take advise such as on Burtons site with a pinch of salt, its guidance thats all. you cannot build a high performance engine by simply buying off the shelf parts, you actually have to measure stuff and work out compressions etc and whats needed to make a package that will work.

PS please stop posting the same thread on different parts of the forum, you wont get any more replies

[Erikmex]

Oh dear!!!! Doesnt sound like anythign has been measured here, a piston advert has no idea what your head gasket thickness is, deck height etc, do you even know that your piston to head clearance is correct? Calculating your compression ratio is one of the most importnat things you must do on a build like this, take it to someone who knows what theyre doing.

[Scotty77]

I have taken to the engine builder

Head gasket is 40 thou, head clearance was fine as i watch this being done. I not the engine builder just trying to find a reason why this is happening as two engine builder can't seem to find the problem.

[katana]

First Rebuild
Attachment 85221

From reading another one of your comments about clearances and looking at the above piston, I would guess clearance / squish is too tight - characterised by lack of carbon in the squish zones. For safety, a minimum of 1mm is good and allows for potential rod stretch - some go tighter but not advisable IMO! Hot spots through fueling / timing leads to detonation and pistons are usually toast before you 'hear' it - the 2nd picture is classic 'det' damage - nitrous abuse is good for making those!

[Graham]

I have taken to the engine builder

Head gasket is 40 thou, head clearance was fine as i watch this being done. I not the engine builder just trying to find a reason why this is happening as two engine builder can't seem to find the problem.

Forgive me for being blunt but your engine builders appear to be the problem!

its simple, too much compression for the cam and possibly the for the octane rating of the fuel you have. any half decent engine builder should be looking at things like actual compression ratio, not going off an advertised ratio, which is a guestimate at best.

[Scotty77]

Forgive me for being blunt but your engine builders appear to be the problem!

its simple, too much compression for the cam and possibly the for the octane rating of the fuel you have. any half decent engine builder should be looking at things like actual compression ratio, not going off an advertised ratio, which is a guestimate at best.

Can you tell me why would it be no 3 and 1 pistons being the worst all the time. Wouldn't all piston go

[Graham]

Can you tell me why would it be no 3 and 1 pistons being the worst all the time. Wouldn't all piston go

they probably would of all gone if you could of kept driving, it could be down to slight differences in volumetry efficiency cylinder to cylinder, slight differences in air flow through your carbs/airbox, maybe cylinders 1 +3 have thinner bores and run cooler, maybe the bores are a fraction bigger on 2+4,

it could be a million things but in case of failure like this, its never all 4 cylinders which end up dead

[Scotty77]

they probably would of all gone if you could of kept driving, it could be down to slight differences in volumetry efficiency cylinder to cylinder, slight differences in air flow through your carbs/airbox, maybe cylinders 1 +3 have thinner bores and run cooler, maybe the bores are a fraction bigger on 2+4,

it could be a million things but in case of failure like this, its never all 4 cylinders which end up dead

Bored have been sleeved, checked sizes yesterday and number 4 was 2 thou bigger the rest of the bored are the same

[Graham]

its not unusual to size the bores bigger as you go to the back of the engine but 2 thou bigger is a LOT.

either way i cant help but feel you are looking at the symptoms of the problem and not the cause. unless your running 120 octane fuel, your combination of cam, compression and ignition timing just isnt going to work

[Scotty77]

its not unusual to size the bores bigger as you go to the back of the engine but 2 thou bigger is a LOT.

either way i cant help but feel you are looking at the symptoms of the problem and not the cause. unless your running 120 octane fuel, your combination of cam, compression and ignition timing just isnt going to work

Can you give me some suggestions using the current pistons on what you would do?

[HonkyWhiteTrash]

Bored have been sleeved, checked sizes yesterday and number 4 was 2 thou bigger the rest of the bored are the same

2 thou bigger than what? If the other bores only have 1.5 thou clearance that could explain why cylinder 4 is OK (ie if it has 3.5 thou clearance). Are any of the cylinders even bored to the right size? It's hard to believe they are with a difference of 2 thou. Are they perfectly round in all directions?

If you want to get a solution you ought to be doing a lot of accurate measuring of fits and tolerances (piston to bore clearance, compression ratio, cam timing etc etc) and not solely relying on an engine shop and the internet for answers. We can advise but you have the evidence - treat it like a post-mortem.

[HonkyWhiteTrash]

I would be wanting to check the spark advance curve and make sure it is correct for your individual spec of engine. In fact, to be sure, I would want to fit a Aldon 103 FX type distributor for that spec of engine. Then you know it's correct.

[Erikmex]

Simply - if youre engine builder has not calculated the compression ratio through measurement and has used off the shelf pistons advertising 12:1, he really doesnt know what he's doing, you really need to take it to someone else, id be charging the builder for the pistons too. Bulding an engine the same way twice sint going to get different results, save yourself money and find someone else.

[Graham]

Can you give me some suggestions using the current pistons on what you would do?

I already have, to start with i would run a milling cutout through the piston valve cutouts and remove the thin wedge shape bits of piston. After that i would get the bureutte the combustion chambers and work out the actual compression. it will probably still be too high, so you just have to work out whether its better to use a thicker head gasket or machine more off the piston. this is all stuff any professional engine builder should do as a matter of course

[Graham]

2 thou bigger than what? If the other bores only have 1.5 thou clearance that could explain why cylinder 4 is OK (ie if it has 3.5 thou clearance). Are any of the cylinders even bored to the right size? It's hard to believe they are with a difference of 2 thou. Are they perfectly round in all directions?

If you want to get a solution you ought to be doing a lot of accurate measuring of fits and tolerances (piston to bore clearance, compression ratio, cam timing etc etc) and not solely relying on an engine shop and the internet for answers. We can advise but you have the evidence - treat it like a post-mortem.

i read it that cylinder 4 had 2 thou more clearance, which means if it the cylinders were bored to the correct size in the first place it will have a massive 6 thou clearance!

[r2vdh]

Does your racing allow you fit a wideband? Could help you see when its happening?