6401BB Build up.

Well I have had this Makita 6401 here for a while now, only cut with it once, and was impressed with it for a 65cc saw. I just liked my 371 better so I always used that. That may change now however when I'm done with this saw, who knows. Got the BB kit in the mail today from Matt aka MCW! Thanks Matt. First look, is yep its a typical BB kit, looks really good, good casting, ports look good. But dang it has small ports and really small transfers. Piston skirt looks pretty narrow, with not much room to widen? Haven't checked how far yet. First things first. Pulled the saw apart, bolted the BB kit on without rings, checked squish, its at .035, so I can do better then that! Over to the lathe I go, take off .015 off the base, to end up with .020, and that's as far as I got tonight. More to come.

Crikey, you are going to have to be creative to get that jug to flow. Take a look at what Timberwolf (Brian) has done on the 361 Shootout thread to see what he did to a stock 361 to get it to flow -

http://www.arboristsite.com/attachment.php?attachmentid=158840&d=1290359711

I'm looking forward to seeing what he does to the 361 BB kit.

I had some private discussion with Matt about where to get a new 6401 and one of his BB kits. Unfortunately, the wife has been out cutting with me and she's seen me successfully cut everything up with the saw I have - her question, "why do you need another saw" (answer: because I want to Honey) put a halt to my CAD. I'll be watching what you guys come up with on the 85cc BB kit.

Matt dropped off the Makita 6401BB today,(thanks Matt) I didn't do any cutting with it today
(too hot over 100F) but I did start it, the compression & throttle response is amazing. My previous newest saw is 1990.. running this one will be a real treat.
Will, I hope you get good results from your BB kit, hopefully these kits will be reliable.

Let her rip Will!

Terry Syd said:

Crikey, you are going to have to be creative to get that jug to flow. Take a look at what Timberwolf (Brian) has done on the 361 Shootout thread to see what he did to a stock 361 to get it to flow -

http://www.arboristsite.com/attachment.php?attachmentid=158840&d=1290359711

I'm looking forward to seeing what he does to the 361 BB kit.

I had some private discussion with Matt about where to get a new 6401 and one of his BB kits. Unfortunately, the wife has been out cutting with me and she's seen me successfully cut everything up with the saw I have - her question, "why do you need another saw" (answer: because I want to Honey) put a halt to my CAD. I'll be watching what you guys come up with on the 85cc BB kit.

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Ya, my understanding is that all 7900's have small transfers like that. BTW, a 288 piston has way wider skirts! But takes a fair amount of work.

gmax said:

Matt dropped off the Makita 6401BB today,(thanks Matt) I didn't do any cutting with it today
(too hot over 100F) but I did start it, the compression & throttle response is amazing. My previous newest saw is 1990.. running this one will be a real treat.
Will, I hope you get good results from your BB kit, hopefully these kits will be reliable.

Click to expand...

AUSSIE1 said:

Let her rip Will!

Click to expand...

Thanks guys, we'll see what we can do with these.

Just for reference Will, here's a Mahle 52mm jug.

Note the transfers are nice and sharp.

The same old problem with many BB kits - increased bore size at the expense of the transfer ducts. I wonder how much the outside wall of the tunnel can be worked before gaps between the fins appear.

If I was a 'fortune teller' I'd say something cryptic like, "I see GB Weld in your future"

You guys are looking for wider piston skirts to open up the exhaust/intake ports. You might first want to check and see if those wider pistons reduce the flow around the piston to the transfers. It would be a bugger to port the jug and then find out the piston restricts the transfers.

This is a great project. Hopefully you blokes will have sorted all this out by the time I convince the wife that I really do need the 6401 and BB kit.

tdi-rick said:

Just for reference Will, here's a Mahle 52mm jug.

Note the transfers are nice and sharp.

Click to expand...

Thanks for the pic, they look nice! These transfers will be getting allot of work.

Terry Syd said:

The same old problem with many BB kits - increased bore size at the expense of the transfer ducts. I wonder how much the outside wall of the tunnel can be worked before gaps between the fins appear.

If I was a 'fortune teller' I'd say something cryptic like, "I see GB Weld in your future"

You guys are looking for wider piston skirts to open up the exhaust/intake ports. You might first want to check and see if those wider pistons reduce the flow around the piston to the transfers. It would be a bugger to port the jug and then find out the piston restricts the transfers.

This is a great project. Hopefully you blokes will have sorted all this out by the time I convince the wife that I really do need the 6401 and BB kit.

Click to expand...

ha ha. I was thinking JB

Narrow piston skirts are a big hinderance on the 7900s. The do have small transfers too. They seem to like a lot of blowdown.

Has anybody taken a degree wheel to these new BB kits?

I'm wondering if a comparison between the time/area of the 64cc, 79cc jugs and the 85cc BB might be informative (or at least some tracings of the ports to compare the areas of the ports, the angles of exit are likely the same). If the time/area of the BB was matched to the 79cc jug, you'd at least know you were in the ballpark of getting the most from the BB.

Do the 7900s have a problem with the narow skirts preventing a wide exhaust port? If so, that may be a factor in the time/area of the blowdown period.

So why does the 7900 have narrow skirts? Is it because of a necessity to get adequate transfer flow?

Don't mind me, I'm just sitting here in the peanut gallery - ran out of peanuts so I'm munching popcorn.

Terry Syd said:

Has anybody taken a degree wheel to these new BB kits?

I'm wondering if a comparison between the time/area of the 64cc, 79cc jugs and the 85cc BB might be informative (or at least some tracings of the ports to compare the areas of the ports, the angles of exit are likely the same). If the time/area of the BB was matched to the 79cc jug, you'd at least know you were in the ballpark of getting the most from the BB.

Do the 7900s have a problem with the narow skirts preventing a wide exhaust port? If so, that may be a factor in the time/area of the blowdown period.

So why does the 7900 have narrow skirts? Is it because of a necessity to get adequate transfer flow?

Don't mind me, I'm just sitting here in the peanut gallery - ran out of peanuts so I'm munching popcorn.

Click to expand...

All good Terry...this is what forums are for.

Al, I find your avatar very distracting, but please don't change it.

Terry Syd said:

Al, I find your avatar very distracting, but please don't change it.

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She is a pretty girl...we build them well here...


With the pin shifted 064 piston I'm getting 65% at the exhaust against 60% with the kit piston. The 064 piston isn't over shrouding the lowers and has windows.

I'm not finding the 372BB's transfers a problem as they open up with little work to help keep velocity.
Compared to the 50mm OEm they are basically the same.
That is the 50mm on the right.



I would tend to think you could have good success with the DolMak 84cc kit.

blsnelling said:

Narrow piston skirts are a big hinderance on the 7900s. The do have small transfers too. They seem to like a lot of blowdown.

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Yar, that's what I hear anyway. I'll keep it as high as I can.

I will be putting a degree wheel on this saw before I do any altering of the ports. But this will be with the squish reduced by .015". I'm going to open up the transfers as much as I possible can. I'm guessing there isn't much room to go wider, on the intake and exhaust. I'll let ya'll know what I find. I'm going to see how the 288 piston will work as well.

Ok, got some numbers here for you junkys! They are pretty conservative numbers if ya ask me. Lots of room for improvement.

Ex-149° duration. 105.5atdc

Int-156° duration.

Transfers 99° duration, 130.5°atdc, also the transfers are quite uneven, that was the highest one, the other was about 2.5° lower.

Blowdown 25°

So I'm thinking to bring the exhaust up to 100° atdc, that will give 160° duration. Bring up the transfers the same amount, that will be 125°atdc about 110°duration.

And keeping the blowdown at 25°

How's that sound?

I think those stock numbers are why Matt is seeing such a torquey saw.

Will, are the transfers staggered uniformly or is it a flaw ?
Staggered transfers can be a good thing, if planned that way.

Anyway, mine should be here in a week or so, so it'll be interesting to compare numbers.

tdi-rick said:

Will, are the transfers staggered uniformly or is it a flaw ?
Staggered transfers can be a good thing, if planned that way.

Anyway, mine should be here in a week or so, so it'll be interesting to compare numbers.

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Ya, know what, I really don't know. What would be the advantage of having one higher then the other? I'm planning on making them even.

Good thread Will. All these numbers don't mean anything to me but I'll be watching closely

MCW said:

Good thread Will. All these numbers don't mean anything to me but I'll be watching closely

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LOL, now's a great time to learn!