stihlboy
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its gonna need the piston crown welded up or you'll freeport
Makita 84cc BB install
- Thread starter bayshorecs
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its gonna need the piston crown welded up or you'll freeport
oil, not water
The only change I would recommend is to use (undiluted) pre-mix oil, not water. Pull 10cc (10mL) into the syringe, piston at TDC, tilt saw so plug hole is plumb, then fill to first thread on spark plug hole. If there's 3 mL left in the syringe, then the combustion chamber volume is 7cc (7mL).
The only change I would recommend is to use (undiluted) pre-mix oil, not water. Pull 10cc (10mL) into the syringe, piston at TDC, tilt saw so plug hole is plumb, then fill to first thread on spark plug hole. If there's 3 mL left in the syringe, then the combustion chamber volume is 7cc (7mL).
mtngun
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You are right, it'll freeport.
It's a low buck project. If the 2nd generation kits were available, I'd go ahead and buy one, but since the 2nd generation kits keep getting pushed out, I thought a pop up would be worth a try.
If it doesn't work, I'm not out much, because the saw is pretty lame the way it is now.
mtngun
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I like your plan. There's a risk of having a bubble trapped inside the chamber, but overall it's probably the most practical way to cc a chainsaw.
AUSSIE1
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mtngun
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Just cc'd my 066BB (and I apologise if we are veering off the Makita BB topic). I did it three times. 11.5, 12.0, and 12.5 cc. That's with 0.020" squish.
The hard part was locating TDC precisely. I used a chopstick through the plug hole as an indicator, which was not super precise. In hindsight, I should have pulled the muffler and determined TDC by observing piston movement through the exhaust port.
The BB's 0.020" squish takes up 1.25 cc. Subtracting the squish leaves 10.75 cc for the combustion chamber alone.
Then I cc'd an OEM 066 P&C. This was off the saw, so I could push the piston all the way up until it touched the squish band. 9.5, 10, 10cc. That particular jug had only blown 135psi with new rings, so I'd say its 10cc chamber is larger than ideal.
The 066BB's static compression ratio is 8.2. The best it has ever blown was 130 psi, and last time I checked it was only 115 or so.
The 066 OEM's static compression ratio would be 8.14 if set up for the same 0.020" squish. As mentioned before, it blew 135 psi when it had a new piston and rings. Makes sense.
I'd like to have 160 psi at my altitude. A little math says that would require a 10.3 static compression ratio. Gotta do a little more math to calculate how much pop-up is needed.
Sorry about hijacking the thread, but I hope the data helps someone. I know it helped me.
Update: Did the pop-up calcs for the BB kit. A 10.3 static compression ratio requires a 9.85 cc chamber, a 2.15cc reduction (BTW the 2nd generation BB kit is supposed to be 1cc smalller).
Let's assume that the maximum practical pop-up is 0.050" high and 35.2mm diameter. That buys 1.24cc, not nearly enough, darn it.
The math is similar for the OEM jug. An 0.050" pop up would only raise the cr to 9.23, yielding perhaps 145 psi at my altitude. Darned ! ! !
My best hope at this point is to wait for the 1cc smaller big bore kit, then install it with minimum squish, ending up with perhaps 150 psi.
The hard part was locating TDC precisely. I used a chopstick through the plug hole as an indicator, which was not super precise. In hindsight, I should have pulled the muffler and determined TDC by observing piston movement through the exhaust port.
The BB's 0.020" squish takes up 1.25 cc. Subtracting the squish leaves 10.75 cc for the combustion chamber alone.
Then I cc'd an OEM 066 P&C. This was off the saw, so I could push the piston all the way up until it touched the squish band. 9.5, 10, 10cc. That particular jug had only blown 135psi with new rings, so I'd say its 10cc chamber is larger than ideal.
The 066BB's static compression ratio is 8.2. The best it has ever blown was 130 psi, and last time I checked it was only 115 or so.
The 066 OEM's static compression ratio would be 8.14 if set up for the same 0.020" squish. As mentioned before, it blew 135 psi when it had a new piston and rings. Makes sense.
I'd like to have 160 psi at my altitude. A little math says that would require a 10.3 static compression ratio. Gotta do a little more math to calculate how much pop-up is needed.
Sorry about hijacking the thread, but I hope the data helps someone. I know it helped me.
Update: Did the pop-up calcs for the BB kit. A 10.3 static compression ratio requires a 9.85 cc chamber, a 2.15cc reduction (BTW the 2nd generation BB kit is supposed to be 1cc smalller).
Let's assume that the maximum practical pop-up is 0.050" high and 35.2mm diameter. That buys 1.24cc, not nearly enough, darn it.
The math is similar for the OEM jug. An 0.050" pop up would only raise the cr to 9.23, yielding perhaps 145 psi at my altitude. Darned ! ! !
My best hope at this point is to wait for the 1cc smaller big bore kit, then install it with minimum squish, ending up with perhaps 150 psi.
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bayshorecs
ArboristSite Operative
Ran the saw for 2 tanks yesterday. The first few cuts were "ok", the saw wasn't running as good as I would like. Then it woke up and took off. I am sure everything started to seat and the straight 2 cycle oil worked its way off.
I checked compression when I was done at 150psi. It was still hot though. I will try to get out and do another cold compression check later this morning.
Wasn't cutting anything too big. Mostly 8"-13" trunks with a 20" bar on it.
I checked compression when I was done at 150psi. It was still hot though. I will try to get out and do another cold compression check later this morning.
Wasn't cutting anything too big. Mostly 8"-13" trunks with a 20" bar on it.
You can expect about 160 cold then.
bayshorecs
ArboristSite Operative
Just tested it cold. 178psi. I guess it did seat right after cutting yesterday!
Reviving old thread...
Ok so just got my 84cc big bore kit (neighbor paid for it for some tree work i did).
Any how I am finding that my 0.035 soldier isn't even getting touched. I "reused" my OEM base gasket and was only getting 120psi cold. From what I've read others have left a gasket out completely. Planning on re-checking compression after about 2-4hrs of heavy cutting yesterday (30in wet basswood). I'm hoping to see at least 150psi with the rings set.
Then I plan to pull the base gasket and just use yamabond. I'm hoping to see 160psi after I do that.
I know there is plenty of squish to play with and was thinking of having 0.015-0.020 milled off. So maybe 200psi after... If the compression is to high I can always install a base gasket.
Anyone done this??? I know there have been tons of us doing this swap, but wondering if anyone has cut their jug down yet??? Results??? The OP said it would void the jug warrenty but for the $90 for the set at northwoods... I'll chance it.
I just don't want the jug flying apart if I jack the compression up to high.
Also, How many turns out on the "H & L" are you guys seeing when tuned in?
Ok so just got my 84cc big bore kit (neighbor paid for it for some tree work i did).
Any how I am finding that my 0.035 soldier isn't even getting touched. I "reused" my OEM base gasket and was only getting 120psi cold. From what I've read others have left a gasket out completely. Planning on re-checking compression after about 2-4hrs of heavy cutting yesterday (30in wet basswood). I'm hoping to see at least 150psi with the rings set.
Then I plan to pull the base gasket and just use yamabond. I'm hoping to see 160psi after I do that.
I know there is plenty of squish to play with and was thinking of having 0.015-0.020 milled off. So maybe 200psi after... If the compression is to high I can always install a base gasket.
Anyone done this??? I know there have been tons of us doing this swap, but wondering if anyone has cut their jug down yet??? Results??? The OP said it would void the jug warrenty but for the $90 for the set at northwoods... I'll chance it.
I just don't want the jug flying apart if I jack the compression up to high.
Also, How many turns out on the "H & L" are you guys seeing when tuned in?
I think 120psi is quite low even before break-in, and a 40psi increase after break-in and ditching the gasket is a lot to expect IMO. Only one way to find out though.
we'll to be honest I think my old $20 cheepo equus compression tester is a little off (or maybe I had the decomp in).
Either way I was more curious to see if anyone had shaved a 6401 BB kit to jack up the compression yet?
Nathan
Either way I was more curious to see if anyone had shaved a 6401 BB kit to jack up the compression yet?
Nathan
so after a little deliberation time here's what I decided (with help)... If I don't have 150psi on my neighbors compression tester tonight (double check between his and mine), and if I have enough room on the skirts of the piston I'll be shaving the cylinder a little.
How much over lap is recommended to keep the saw out of free porting? anyone know?
How much over lap is recommended to keep the saw out of free porting? anyone know?
watsonr
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My kit from two years ago has the same problem..... 120psi, no base gasket and over .035 squish! Saw runs like a banshee, just not on high compression. I hardly use the saw and even tried to sell it a couple times, but because of the "low compression" know body wants the saw except me. Steve, its a number and if that saw runs like garbage, I'd be worried. If it runs good (mine throws chip 20') and your not selling it, run it and smile!
I do have a good 79cc cylinder that I took off and could just put it back on with a new piston... but why, it's running really good.
I do have a good 79cc cylinder that I took off and could just put it back on with a new piston... but why, it's running really good.
SawTroll
Information Collector
My brother bought 2 different 84cc kits (one from Baileys), planning to port one of them, and put on a 6400 he bought used on Ebay. He is pretty "educated" on porting two-cycles, and concluded rapidly that none of them were worth the trouble, and neither would make nearly as much power as the OEM 7900 when stock (he also has a new 7900, that is stock until further). He is getting a 7900 one for the 6400, as it has more potensial, and leave the 84cc ones as paperweights. The main problem (in his opinion) is that there isn't enough material in some critical spots for really good porting - and this is better on the OEM 7900 jug, although not at the level of some Husky cylinders, like the 372 and 385/390 ones.
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How long ago was this? I recently put a NWP 84cc kit on and find it a bit hard to believe it wouldn't give an OEM 79cc a pretty good run in stock form. This new BB kit seemed to have a lot better quality than the older ones. Compression w/o base gasket seems to be higher than my OEM kit ever was.
SawTroll
Information Collector
Within the last few weeks - one of the kits was a NWP one.
watsonr
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What is the compression?
The new NWP kit is 185psi cold and having had several tanks of run time. I've always had suspicions my old comp tester was not accurate but useful only as a guide. My new tester seems a bit more accurate. When I assembled this saw it was obvious that the compression was GOOD, without the de-comp engaged it's a real bear just to pull. The OEM 79cc kit never felt like this even w/o base gasket.
