Porting the 361 Big Bore

[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...

 

[blsnelling]

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.

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.

 

[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 on here.

 

[volks-man]

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.

:agree2:
thanks!

 

[Lakeside53]

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?

 

[brncreeper]

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

 

[parrisw]

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

 

[mtfallsmikey]

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

:agree2: :agree2:

 

[UK heartwood]

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

 

[blsnelling]

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.

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.

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.

 

[Lakeside53]

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

:agree2: :agree2:

HUH? Nothing I did here was praiseworthy:greenchainsaw:

 

[spacemule]

HUH? Nothing I did here was praiseworthy:greenchainsaw:

He meant to say spacemule.

 

[mtfallsmikey]

Sorry...BF

 

[blsnelling]

The cylinder is done. This is the first time I did much work to the transfers. I hesitate to even show them. I don't really have the proper tools. I do not have a right-angle grinder. Instead, I had to use a cutoff wheel and a round semi-soft wheel about 1/4" thick. They're not pretty, but it's better than I was afraid they might be. I think they'll flow fine. Notice how I brought the rear transfer back towards the intake more.

 

[056kid]

I dosent look like you did it with a cut off wheel!!!! it looks pretty drn good!

 

[FELLNORTH]

Are You Working Over The Crankshaft As Well. I'm Not Real Specialized In Small Engines ( I Normally Play Car Stuff ) But Taking Weight Off The Piston And Wrist Pin Will Create An Imbalance In Your Engine, And At 14k That Cant Be Good.

 

[blsnelling]

I dosent look like you did it with a cut off wheel!!!! it looks pretty drn good!

I finished with the semi-soft 1/4" wheel. I have no idea what it is, just glad I had it. I need order a bunch of different stuff from McMasterCarr and then I'll know what I want to reorder in the future.

 

[FELLNORTH]

beautiful portwork by the way, that cant be easy, i have a hard enough time with intake port of a small block ford....

 

[blsnelling]

The porting work is done. I'm just waiting on the circlips from Gregg. I believe he dropped them in the mail yesterday. Unfortunately, that's on the west coast. So it'll probably be next week before I get to satisfy my curiosity as to the results

 

[timberwolf]

Ports look good.

But Taking Weight Off The Piston And Wrist Pin Will Create An Imbalance In Your Engine, And At 14k That Cant Be Good.

Unlike a multi cylinder engine, a two stroke single can not be balanced, at least not without some sort of counter shaft/weight.

lowering the weight of the piston will only reduce the inherant imbalance a little. The horizantal imbalance created by the crank bells moving weight front to back will be the same, the vertical imbalance will be reduced though as the piston weight will be lower and as the crank bells only balance about half the piston weight there will be less net imbalance in the up down direction.