Bridgeless transfer ports on Stihl 026 / 260 oem

[NickC]

Greetings. I'm after opinion as to why and the benefits / negatives of the new port design. Just curious as I bought a few kits and one is Stihl Mahle, the other are genuine Stihl no maker from Brazil but come with Mahle pistons.

I'm not surprised that they are getting these made for old models from cheaper sources, I've seen the same port change on an 024 Meteor kit. I assume it's cost and a cheaper, easier casting bit if that's the case why don't the Chinese kits do it? I guess they just copy Mahle and cost isn't an issue as they put no effort into getting the casting right.

Just wondered if any of you fellas that are into porting have any opinions or experience with them, as that is what I'd like to do... If they're no good for that I'll move them on stock.

Thanks!

 

[82F100SWB]

Very interesting, even moreso that current production 260's(non north American market) have not changed power ratings. Definitely a cost saving venture though.

 

[pioneerguy600]

Open port cylinders have mostly been less powerful than closed port, havn`t seen any of these open port jobbies yet but would venture they will be less power full than the old OEM ones were.

 

[Ketchup]

Looks like a step backwards to me. Almost certainly a production cost scheme.

There are a few known examples of excellent open port transfer cylinders (200t, 930super), but most of them are quad ports, not dual. That transfer reminds me of a 1980’s clamshell. With a big enough carb and a tight enough case it might make some power, but on a 260, I can’t see how.

Any other changes? Timing? Squish?

 

[NickC]

Thanks. I haven't had a chance yet but will have a measure soon, it is a different casting rather than just deleting the 'bridge'. They've put some effort into the shaping of the transfer to direct the charge.

 

[NickC]

I've had a look at it, so far as I can tell the timing is as near as damn it the same accurately measuring from the cylinder base, the transfers are a bit wider and perhaps a little deeper (more volume) The squish is less, Mahle ex side 0.52mm in side 0.55mm (0.021"), Brasil 0.77mm ex side 0.71mm (0.030") in side. I haven't compared compression but the chamber looks similar, though I did notice the effective squish band where the solder flattened was significantly wider on the Brasil jug. The in and ex ports are the same heights but a little tighter, setting a bore gauge to drop in the port from the bore on the Mahle wasn't even close to dropping in the Brasil jug. the outer size and shape of the ports are the same.

So all in all not too promising, a bit annoying as I thought I was buying all Mahle but at the price I paid I can move them on easily I'm sure.

 

[NickC]

Has anyone got any good timing numbers for an 026?

I'm going to have a go at one of these Brazil pots, got my depth gauge working and found that the outer ring is a bit higher by 0.25mm (less squish) but the roof is about 0.25mm lower and a close match to the piston crown. There is very little step compared to the Mahle.

I've cut the base to get the squish down, will fettle the ports and having sourced a right angle handpiece for a Foredom I fancy having a proper go at it......

 

[Yotaismygame]

I would assume less case compression with less material being in the jug. I’ve only ported one dual open port saw and its a beast.

 

[Yotaismygame]

Never touched that saw before but find out what the numbers are stock and I could give some input

 

[Huskybill]

I cut the rib between the ports as far as the cutter will go. Then I make a upside down half moon shape facing the crankcase with the rib. This sharp knife shape upside down moon shape keeps the fuel mix more centered in the transfer ports.

 

[Ketchup]

I cut the rib between the ports as far as the cutter will go. Then I make a upside down half moon shape facing the crankcase with the rib. This sharp knife shape upside down moon shape keeps the fuel mix more centered in the transfer ports.

Uh...
Got a picture?

 

[82F100SWB]

I should throw a degree wheel on the last 026 I built. I may have winged it and only measured changes in port heights, there may have been whiskey and educated guesstimation based on previous builds involved, but she does run damn good.

 

[Huskybill]

When I started porting bikes I tried one change at a time on each engine to see the difference. Then I did all the changes on one engine. It was a beast. I copied what was done on the race 2100.

 

[Yotaismygame]

I should throw a degree wheel on the last 026 I built. I may have winged it and only measured changes in port heights, there may have been whiskey and educated guesstimation based on previous builds involved, but she does run damn good.

that’s a hot saw. I’d like to hear the numbers just out of curiosity. Nicely done chain too

 

[NickC]

Thanks guys! That saw does rip...

I'm about to make a mount for the degree wheel so will report back with stock numbers.

I've cut the base and got squish of In - 0.020, Ex - 0.021, Left - 0.019, Right - 0.020. These are similar to the Mahle stock, I didn't go any tighter as the squish band is do much broader than the Mahle I thought I'd see what I get at that, see photo of solder comparison. I did tape some paper on top of a piston and clean up the roof which mostly got rid of the little step.

 

[NickC]

Ok, got the wheel mounted and the numbers for the decked Brazil jug which is to be ported are as follows.

Exhaust open - 97° atdc
Trans open - 121° atdc
Intake open - 109° abdc

Am I thinking about the numbers in the right way? As a rotation / cycle.

The stock Mahle is, 95°120°112° which suggests the porting on both cylinders is effectively the same.

I have read a lot about the subject but I'm trying to resist the temptation to just smash into it, so if anyone has some numbers or ideas I'm all ears.

 

[Yotaismygame]

You’re doing something wrong. The exhaust could be 97 but that’s really high for a small saw. The intake doesn’t make sense. Should be 60-80 or so

 

[Yotaismygame]

Are you measuring by tdc and bdc or duration? Personally I like using the duration method

 

[NickC]

Running rotation of the crank, clockwise rotation of the wheel.

So exhaust opens 97° after tdc, followed by transfers, intake opens 107° after BDC.

 

[Yotaismygame]

Where are you getting 107? Try the duration method. Rotate piston down until it barely opens on the exhaust side. Set your wheel to zero. Rotate it down through bdc and back up until it’s barely open. Read the wheel. Should be around 150 duration. 150/2=75. 75-180= 105 exhaust height. You’ll have to count up once you past 90 on your wheel. I’ve got mine all marked up so it’s easier to read.

you may know this stuff already. Just trying to help