7900 BB cylinder pics

[nmurph]

i recently picked up another 6401. i wanted to have one to put the BB kit on and compare to my other 7900's. Big Brown delivered the package to me friday and when everyone went beddy-bye, i opened the box to have a look. this is what i found.

i have a couple of questions as a result:

is the nickasil (i'm guessing this is what they use on this cylinder) honed after it is applied?

if so, does it look like the cylinder is out of round bc the hone did not touch the cylinder in the area around the decomp port.

would you run this cylinder or ask for a replacement? don't tell me to get an OEM cylinder. i already have done two conversions using them and this one was strictly for funzeees and to have a BB to compare to the 79cc version.

 

[tdi-rick]

Yep, Nikasil is honed after plating.

Does the ring go that high ?

IIRC the ring land is something like .130-.140" on the OE piston.

I have it written down somewhere, so add the squish to that and then measure how far down that decomp port is, I'm pretty sure it's above the ring.

 

[nmurph]

i was wondering if the rings went up that high?

 

[Jacob J.]

I'd do an exchange on that one. The transfer ports look to be a bit off as well.

 

[nmurph]

JJ, do you mean shape???
here are a couple more pics.....the one through the intake shows a nick in the cylinder wall.

 

[parrisw]

I'd do an exchange on that one. The transfer ports look to be a bit off as well.

I was thinking the same thing JJ, when I saw those transfers. If it were me, I'd ask Grande Dog if you could exchange it for another one.

 

[tdi-rick]

The shape isn't great, but you can fix that when grinding them, and it looks like they have staggered timing, although it may be an illusion due to the angle.

 

[mtngun]

Thanks for the pics, Nmurph.

I'm glad you checked the top of the bore. It seems to be a known weakness on the kits.

It would probably run OK, but ....... it's sure hard to work up any enthusiasm for using that jug.

I'm hoping that if enough people post pictures like that and send the jugs back, the manufacturer(s) will get motivated to correct the problem.

 

[nmurph]

i will get some more pics of both sets of the transfers. i can take care of the shape if they are a little off.
does anyone else think the top of the bore may be out of round bc of the way the hone ran on the nickasil? if i could show you a pic, the nickasil is honed opposite of the decomp passage. so i know it not that they stopped honing bf they got to that point in the cylinder.

 

[mtngun]

does anyone else think the top of the bore may be out of round bc of the way the hone ran on the nickasil?

Perhaps Grande Dog will explain what causes the missing plating ?

Presumably they have some way to control which part of the jug gets plated and which part does not. Something seems to be going wrong in the process.

 

[Anthony_Va.]

i will get some more pics of both sets of the transfers. i can take care of the shape if they are a little off.
does anyone else think the top of the bore may be out of round bc of the way the hone ran on the nickasil? if i could show you a pic, the nickasil is honed opposite of the decomp passage. so i know it not that they stopped honing bf they got to that point in the cylinder.

I'd bout think it has to be out of round of the hone missed parts like that. Would be different if they had'nt went that far honing but you say it's hones on the other sides.

There's a few too many what if's on this one to keep it IMO. Knicks in the cylinder walls. Difference in the transfer shapes(they look really weird in the pic). Out of round. Send her back for a better one.

 

[tdi-rick]

Not so much out of round as lacking plating thickness in that spot.

I'm guessing there hasn't been enough material deposited, for whatever reason just there so the hone's missed it.

 

[nmurph]

Not so much out of round as lacking plating thickness in that spot.

I'm guessing there hasn't been enough material deposited, for whatever reason just there so the hone's missed it.

i think you may be right about this.......

 

[nmurph]

more BB cylinder pics

The shape isn't great, but you can fix that when grinding them, and it looks like they have staggered timing, although it may be an illusion due to the angle.

here are some more pics of the transfers. their shape is pretty crude. that could be taken care of, but then i was really wanting to compare stock with stock.

 

[nmurph]

intake/exhaust ports

 

[rmihalek]

That looks pretty similar to the BB kit I got a couple months ago for my 6401. I installed it anyway and resisted the tremendous urge to clean up the ports...I didn't want to void the warranty if it didn't run well. Mine had a horizontal gouge-type defect that almost connected the two transfers on one side; looks like yours has a small indentation in this same spot.

Turns out the saw runs like a champ. Next year, after I've run 10 or 15 gallons through it, I'll tear it down for some measurements and clean up/open up the ports a bit.

 

[nmurph]

bob, how was the plating on your BB? that is what concerns me more. i can clean up the ports with a few minutes on the bench.

 

[straight6jeff]

after looking at the pictures, it looks as though it was shot-blasted in a few spots. Seems like rather poor quality.

 

[ECRUPPRECHT]

I dont have a whole lot of experience with rebuilding saws or removing jugs on saws, i am a certified mechanic, but to me that looks like S**T very poor quality and probably made for pennies?? that would never pass on any engine ive seen?? JMHO but i dont know

 

[AUSSIE1]

We've seen this with the upper transfers with previous 7900BB cyl's so the next cyl will probably look the same in this respect. For those that do work to there ports this wouldn't be an issue. Another cyl may be cleaner in the nikasil though.