OzVMX Forum
Clubroom => Tech Talk => Topic started by: Tony.Brown on March 14, 2017, 06:52:55 am
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I hope someone here can help with some expert advice.
While assembling my 1988 Husqvarna 250WR, I cleaned up the case to transfer port transition, but when I slide the piston into the cylinder it seems to have an excessive amount of piston skirt protruding into the transfer port area in the sleeve opening.
Is this something that I could benefit from by matching the two?
It just looks wrong. I know the piston spends very little time at BDC but it is counter intuitive to a smooth flow through the transfers.
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/IMG20170313200140_zpszndltww6.jpg)
Tony.
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AJ, there are plenty of really experienced people on here but for my part I would not be touching the bottom of that piston, because over the years I've seen too many pistons cracked starting from the top corner of the cutaway on the bottom of the skirt, and on your piston you can see those "strengthening bars" on the inside which are probably there to stop that sort of cracking
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It would be mechanically safe to remove the material; in fact it would make the piston stronger by lightening it. That is probably not the point though. That skirt width is probably a trade off between transfer/case flow and preventing leakage to the exhaust port.
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Thanks Marcus, that makes sense.
This is a long stroke 88 250 Husky, it seems that there is plenty of material between the skirt and the exhaust port, I'm just not sure if there is a tangible benefit in doing it.
I've already removed material on the cases to match the transfer ports (and base gasket) in the cylinder, over 5mm in places!
Maybe that should be enough?
Tony.
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Give it a try, it is a sturdy looking forged piston. Who knows where in the cycle the transfer stream lining is important.
OT I bought a Pre70 TS250 Savage a couple of weeks ago that has had the piston modified at the other end to improve flow: the crown has been carefully reshaped to advance the transfer opening.
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I hope someone here can help with some expert advice.
While assembling my 1988 Husqvarna 250WR, I cleaned up the case to transfer port transition, but when I slide the piston into the cylinder it seems to have an excessive amount of piston skirt protruding into the transfer port area in the sleeve opening.
Is this something that I could benefit from by matching the two?
It just looks wrong. I know the piston spends very little time at BDC but it is counter intuitive to a smooth flow through the transfers.
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/IMG20170313200140_zpszndltww6.jpg)
Tony.
There is no benefit. End of. This is how a 53hp at the wheel 125cc GP cylinder looks at BDC. As I said 100 times before nothing really "flows" through the transfers, more intake charge "parks" under the transfers then in the transfer ducts then finally into the cylinder. The cylinder here is how the transfer duct bore junction should look
(https://s-media-cache-ak0.pinimg.com/564x/83/b4/c3/83b4c32e28d033510f1a587f0a00a3fb.jpg)
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"As I said 100 times before nothing really "flows" through the transfers"
Please excuse my ignorance Lozza, I haven't taken the time to read your previous 100 posts on this subject. Despite searching for previous posts on this subject to the best of my ability, I found nothing. That's why I asked the question.
If it does not flow through the transfers, how does it move through the ports to the combustion chamber? Teleport?
Tony.
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I hope someone here can help with some expert advice.
While assembling my 1988 Husqvarna 250WR, I cleaned up the case to transfer port transition, but when I slide the piston into the cylinder it seems to have an excessive amount of piston skirt protruding into the transfer port area in the sleeve opening.
Is this something that I could benefit from by matching the two?
It just looks wrong. I know the piston spends very little time at BDC but it is counter intuitive to a smooth flow through the transfers.
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/IMG20170313200140_zpszndltww6.jpg)
Tony.
Is that an original piston or an aftermarket one?
I'm not going to comment on any of the above answers some are a bit strange.
I'd check on how much overlap there is on both the inlet and the exhaust ports and make sure the skirt has about 6mm either side. If you do feel like taking some out don't go any further then the raised area on the piston inside as they are there for strength.
You will notice on the pretty picture above that the even though the cutaway is not a match at BDC it is quite close with the piston up the bore a bit allowing good flow into the port with the piston up a little.
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(http://i753.photobucket.com/albums/xx172/mike52_photo/Many%20jiff/smiley-face-popcorn1.gif) (http://s753.photobucket.com/user/mike52_photo/media/Many%20jiff/smiley-face-popcorn1.gif.html)
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I hope someone here can help with some expert advice.
While assembling my 1988 Husqvarna 250WR, I cleaned up the case to transfer port transition, but when I slide the piston into the cylinder it seems to have an excessive amount of piston skirt protruding into the transfer port area in the sleeve opening.
Is this something that I could benefit from by matching the two?
It just looks wrong. I know the piston spends very little time at BDC but it is counter intuitive to a smooth flow through the transfers.
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/IMG20170313200140_zpszndltww6.jpg)
Tony.
Is that an original piston or an aftermarket one?
I'm not going to comment on any of the above answers some are a bit strange.
I'd check on how much overlap there is on both the inlet and the exhaust ports and make sure the skirt has about 6mm either side. If you do feel like taking some out don't go any further then the raised area on the piston inside as they are there for strength.
You will notice on the pretty picture above that the even though the cutaway is not a match at BDC it is quite close with the piston up the bore a bit allowing good flow into the port with the piston up a little.
Hi sleepy, it's a Wossner, the only piston available for this model, originally from a 240, 1mm larger than the unavailable std piston. I still need to cut away the rear piston skirt to match the original.
Tony.
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Without seeing the piston in the flesh I'd say you could safely take a bit out without effecting the strength. If you can get it close to the OEM piston in shape the motor should work as designed.
You will probably be told shortly that Wossner piston are rubbish but like the vast majority of happy users just ignore that BS.
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woosners just need an extra thou or so clearance than stated from what I can deduce. plenty of chat re this on café husky. probably only applies to air cooled models from what I can see but careful run in and lots of hot cool cycles is my thoughts
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woosners just need an extra thou or so clearance than stated from what I can deduce. plenty of chat re this on café husky. probably only applies to air cooled models from what I can see but careful run in and lots of hot cool cycles is my thoughts
Yeah, I have to agree. I've run Wossner's in the past with zero issues.
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Hmmmm.........
It seems I didn't explain my question clearly enough, thank you Lozza for the photo that perfectly demonstrates the result that I was asking about!
The areas I've highlighted (in yellow) are the areas of interest to me, I won't touch the reinforcing ribs but this skirt effectively reduces the size of the transfer port from left to right, the area directly below the piston pin (gudgeon) is to be left alone. Not all of the material can be removed but some major improvements could be made in this area. I believe the sleeve/cylinder was designed this way for a reason, if it made no difference, why would Husqvarna cut the sleeve in such a specific shape?
Bottom line, if it makes no difference......who cares? It isn't going to do any harm.
But........if it's good enough for 53 RWHP on a 125, it's good enough for this old bugger!
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/GP%20cylinder_zpsy4xrk6fk.jpg)
(http://i1276.photobucket.com/albums/y464/ajcmbrown/Husqvarna%20250WR%201988/250WR%20piston%20skirt_zpsleakxkw3.jpg)
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"As I said 100 times before nothing really "flows" through the transfers"
Please excuse my ignorance Lozza, I haven't taken the time to read your previous 100 posts on this subject. Despite searching for previous posts on this subject to the best of my ability, I found nothing. That's why I asked the question.
If it does not flow through the transfers, how does it move through the ports to the combustion chamber? Teleport?
Tony.
Its a 2T engine if something seems logical it's the least likely thing to happen. I say "nothing flows through the transfers" because its a multi step process for intake charge to go from carburetor to combustion chamber. First in into the crank case where it stays for a few cycles, then under the transfer ducts, then into the transfer ducts, then finaly into the combustion chamber. It is not a 1 step process, which is why altering the piston to match the cylinder spigot will make no difference.Hmmmm.........
It seems I didn't explain my question clearly enough, thank you Lozza for the photo that perfectly demonstrates the result that I was asking about!
The areas I've highlighted (in yellow) are the areas of interest to me, I won't touch the reinforcing ribs but this skirt effectively reduces the size of the transfer port from left to right, the area directly below the piston pin (gudgeon) is to be left alone. Not all of the material can be removed but some major improvements could be made in this area. I believe the sleeve/cylinder was designed this way for a reason, if it made no difference, why would Husqvarna cut the sleeve in such a specific shape?
Bottom line, if it makes no difference......who cares? It isn't going to do any harm.
But........if it's good enough for 53 RWHP on a 125, it's good enough for this old bugger!
Your thinking in a static sense, the crankcase on a two stroke is not a static environment. The crank is spinning and the piston is going up and down through the centre of all that is the rod. The intake charge takes a series of small steps toward the combustion chamber. Logs or rocks in a river don't stop the river flowing. The most important aspect of a 2 stoke is what happens to the intake streams once they come out of the transfer ports. Grinding the piston away won't amount to any improvements. Improvements come mainly from outside the engine pipe, ignition and jetting.
Only Husqvana knows why they did that, even then its down to who is ever sitting in the chair at the time.
.............and Wossners are still as shit as ever
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LOZZA- What is the problem with Wossner pistons?.
What are the best Piston Brand or Brands ?.
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Oh dear, you better start reading 8)
http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf
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I think it's best to remember that a two stroke engine is nothing more than an air pump, all of the things that take place in a two stroke engine happen in one revolution of the crankshaft, or two strokes, hence the name. Not several cycles.
With air/fuel flowing through the engine at high speed, any disruption of the flow will be to the detriment of performance, with the exception of a properly designed expansion chamber, which is designed to provide back pressure/resonance to prevent un-burnt gases going out the exhaust port instead of being burned in the combustion chamber.
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Oh dear, you better start reading 8)
http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf
Agreed.
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Logs or rocks in a river don't stop the river flowing.
If they build up high enough they become what we call a dam - ask a beaver. They certainly affect the flow, have a look at what the water does around a log, which is the best place to catch a fish - ask a bear.
The intake charge flows in a motor dependant upon the various changes in pressure, which are gradual changes, not step changes.
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(https://s3.amazonaws.com/WEBPOSTS/OZVMX/chilling.jpg)
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he needs to stop riding Hondas and get on a husky, will stop pounding his castanets on the tank! 8)