Thread: Porting the 361 Big Bore

Originally Posted by Lakeside53

None that I'm aware of... My packets of clips around that size are all the same wire. Maybe Husky?

i spent tooo much time earlier looking for one of thoes stupid clips.


You know usually when you loose a snap ring or anything of the type, there is a little "tingg" when it lands. There was no sutch noise so i looked where ever there was soft material except my jacket which was at arms length. i figured that i went pretty far.

it was chillen on my jacket which i was just about to yank up and vacate due to anger. Lucky i saw it!

Exhaust 100* / Duration 160*
Transfers 120* / Duration 120*
Intake 75* / Duration 150*
Blowdown 20*

Well, these are the numbers I'm going, I mean have gone with, lol. The intake and exhaust are done. I still have to do the transfers and piston.

A lot of you guys have been asking about porting. I'm no expert, but here's what I do.

1. Begin by measuring compression, squish, and port timing. That's a whole topic in itself that I'm a total rookie at. I can get the job done, but that's about it.
2. .020" is the general rule of thumb for safe squish. The larger the saw, the greater the squish will need to be. For example my 260 is at .015" and my 084 at .021". No signs of detonation or errosion.
3. I take both the intake and exhaust as wide as the piston skirts will let me. You have to watch out for ring end locations as well. There must by cylinder wall for the ring end to ride on as it goes past the port window.
4. Port shape is very important. Make the sides of the intake and exhaust straight. Make the roof of the exhaust and floor of the intake "flattish", but make sure there's a slight radius so that the ring is tucked back in gently and dooesn't snag. Put the piston in the cylinder and check your work. The port should close from the sides to the center. The floor of the exhaust and roof ot the intake are not that critical since that's not where flow initiates.
5. I angle the back of the rear transfers back towards the intake side and clean them up in general.
6. Most Pro Stihls have a lower transfer intake that I taper all the way to the bottom of the jug. The 361 doesn't have those so you won't be seeing them here.
7. Put a radius bevel on the edge of all ports. This is critical so as to not catch a ring and destroy your work.
8. Port and flow the windows in the piston. Create angles that promote flow from the inside of the piston to the outside. A lot of material can be removed, but take care not to hurt the structural integrity of the piston.
9. Port match the exhaust gasket, heat shield, and muffler inlet.
10. Mod the muffler such that total exhaust oulets is between 125%-150% of the area of the exhaust port at the cylinder wall.

I'm sure I'm missing some stuff here, but you should get the general idea.

Here's some pics of the 361BB in progress.

Intake Before and after. Notice that I use tape as the guide for where I will port to.


Port finished but not beveled.


I wanted to raise the exhaust port .055" for a 5* change. I cut tape that wide and laid it along the port edge.


I laid another piece under it that I used as a porting guide. You may notice that I took more off one side than the other. That was by design to leave plenty of room for the ring end to ride on.


Here's a shot to give you an idea how much material I removed at the flange. Notice that I did not lower the floor at all. The scribed line is from my previously modded heat shield.


Exhaust port finished and the piston installed to check for a slight radius as it closes.

Notice the straight sides. Again, this port has not yet been beveled.

dank!

Awesome work, thanks for all the great tips. I was trying to figure out a way to check skirt clearance, never thought to mark on the piston itself..nice!

Also wasn't sure on the best way to guide/mark inside the cylinder, I tried a pencil, but that wasn't very precise, tape is a good idea too.

I also didn't think about the ring gap.

Any tips on or pics of the cutting tools you're using, looks like some sort of die grinder followed up with some needle files/hand work for smoothing?

Good stuff..

Also any general timing numbers or guides? I think I figured out how to figure out what timing I have, but I'm lost as to what I should be looking for.

I know it's all pretty complex how it works, but I like the rules of thumb or guides..gives newbs like me a chance to try and understand...

Originally Posted by Martinm210

Any tips on or pics of the cutting tools you're using, looks like some sort of die grinder followed up with some needle files/hand work for smoothing?

I use a double-cut, cylinder radius tipped carbide burr in a Dremel, 1/8" shank, 1/4" cutter. Use a ball shaped stone to bevel the port edges.

Originally Posted by Martinm210

Also any general timing numbers or guides? I think I figured out how to figure out what timing I have, but I'm lost as to what I should be looking for.

I know it's all pretty complex how it works, but I like the rules of thumb or guides..gives newbs like me a chance to try and understand...

Timing rule of thumb, leave timing stock. That's how most of my saws are and it works well on them. This BB cylinder is the first time I ever raised an exhaust port like that.



I put the P&C back on the saw to check a couple things. The exhaust is at exactly 100* as I planned. Woohoo. Like I said, this was the first time I drastically changed port timing. Somehow I managed to get a couple degrees out of the intake so it's at 73* now. I'm happy with that. I didn't think I could get that with that little passage in the way. All that's left are the transfers. Then need to go up to about 120* or I'm likely to be starved for fuel at high RPMs and it then wouldn't hold RPMs at the top.

Here are a couple shots of the piston. Nothing pretty, but it'll flow a lot better. Lighter is good too when you're turning more RPMs.

Thanks Brad for taking the time too show us how you do your porting. It looks like a safe approach for a rookie like me. So few people will give specifics about thier porting because they either race, or they do it for a bussiness. It is appreciated by alot of folks on here.

Originally Posted by rbmopar

Thanks Brad for taking the time too show us how you do your porting. It looks like a safe approach for a rookie like me. So few people will give specifics about thier porting because they either race, or they do it for a bussiness. It is appreciated by alot of folks an here.

thanks!

Originally Posted by blsnelling

Here are a couple shots of the piston. Nothing pretty, but it'll flow a lot better. Lighter is good too when you're turning more RPMs.

Can you use the OEM pin to loose more weight?

Brad, could you post a pic of your porting tools, bits, dremel, and holding vice/stand?

Hey, Brad, really nice work. How do you accurately cut and measure the tape to .055"

Very educational!!....Kudos to Andy and Brad!!!

+1 great pics just what I needed to get started.

Originally Posted by Lakeside53

Can you use the OEM pin to loose more weight?

Absolutely. I used the BB pin the first time round. I'll have to post a shot to show the difference.

Originally Posted by brncreeper

Brad, could you post a pic of your porting tools, bits, dremel, and holding vice/stand?

I can. Nothing but a cable drive Dremel and a chainsaw file. No vise, no fancy tools.

Originally Posted by parrisw

Hey, Brad, really nice work. How do you accurately cut and measure the tape to .055"

I just set my caliper to .050, marked it, and cut it. If it was off a hair it wouldn't matter much. It takes about .011" for each degree. I wasn't worried about ending up at 99*. It turned out dead on though.