Linear measurement for port durations

[David Young]

Hey all

I got a lot of requests for a thread on how to do this. I think paragon builder ran with it and did a nice job in the 346 porting first time thread.

I want to put all the pics and info in the front so please wait to reply.

I have some pictures from a ms261 that suffered a bearing failure. So the squish came in at 40 thou. I think it is normally 32 thou. So one this saw it would mean my durations would be off by 1 degree.



First step is use a lip of the cylinder and measure to the squish band. Then zero out the calipers.

 

[David Young]

Next I put a ring in the bore and use a piston to make sure it's square.

Then measure the distance toting or port opening.

23.66 to exhaust but we need to subtract the squish. So 22.66 for 40 thou or 22.84 for 32 thou which is .82 mm

 

[David Young]

Transfers. Same mathing going on here

 

[David Young]

Just numbers

 

[David Young]

Here is how I get the rod length. Not exact but not a huge influence in the out come from my experience

I'm just going to round these numbers.

 

[David Young]

Here are my numbers hopefully this makes sense.

 

[David Young]

When a plastic caged bearing goes bad. It stuck in the chamber

 

[mdavlee]

I'll have to try this on the next one I do just to see how close they come to the wheel.

 

[David Young]

With in a half degree Mike but part of sharing is. I'd love to see other tweaks on this and people show me where this can be in error.

I have domestic duties to attend to. I'll be back today with how i do the intake and the software use.

 

[Magic_Man]

Here are my numbers hopefully this makes sense.

Maybe I'm missing something here, what formula did you use to come up with those numbers ?

 

[Stihlbro]

Maybe I'm missing something here, what formula did you use to come up with those numbers ?

Wait for it, he stated he had domestic duties........


Sent from my iPhone using Tapatalk

 

[David Young]

The program is free at torqsoft.com On the left menu you can see piston position.

I have attached three photos. The first does not take into account the squish clearance.
Second takes into account measured squish.
Third is what I suspect the clearance would be if the bearings were good.
I actually have another 261 I measured.


David

 

[Magic_Man]

Ah, now I see, Thanks !

 

[Mastermind]

I'm not a smart man.

I'll have to keep using the wheel.

 

[mt.stalker]

I'm not a smart man.

I'll have to keep using the wheel.

You have a nice side business , a great wife and family ,...... I'd say ur pretty smart

 

[Magic_Man]

I'm not a smart man.

I'll have to keep using the wheel.

This program works perfect with your card stock method for grinding port height.@Mastermind

 

[Mastermind]

I keep up with port height.....but I still double check with the wheel.

 

[Tor R]

Awesome thread David!!

@Mastermind, would not this thread fit well in technical area?

 

[ft. churchill]

I've got a new co-worker, who was a porter of two stroke road race bikes as well as dirt bike. At least up until those eras ended a few years back. They did all of their work by measurements of port heights and widths, he was fascinated by our degree wheel method. ( He was the builder of the two stroke yamaha 350 that held the fastest lap at Willow Springs Raceway for a period of years).

 

[Deets066]

This method would work well to check numbers while porting if you don't use the card stock trick. Or to double check before grinding incase your wheel happened to move while setting desired port height.