I haven't tried the 038 or 390 setup yet. Something I would like to do at some point. But have built a few 660's and found a few things I like to address in the build stages with the MS660's. The saws I have built have run very well to this point. Pretty impressed overall. I did have to address a couple of things and some might be relevant to this discussion.
1) I use the OEM decomp, pulse line, and chain adjusters for a variety of reasons.
2) On some of the MS660 56mm BB cylinders I had to "deburr" where the spark plug hole intersects the combustion chamber. A little "flashing" sometimes is left in there and could break off.
3) On the 56mm BB's you need to check and see if the piston skirt interferes with the base gasket. You will feel it when your doing your squish test and rotate the piston through BDC without the piston ring....with the piston ring in as well I guess just I always do a check w/o ring to see how much squish the build has and also to be certain I have the clearance at BDC. I used a dremel tool or die grinder to open those gaskets just a little.
4) There is a potential for free porting on the exhaust on any brands 56mm BB kit for MS66o's , Farmertec is no exception. It's a geometry thing as a 56mm piston is not going into the cases. SO a shortened piston skirt is the answer and that potentially leads to a Freeport (exhaust and intake open to the crank case at the same time) issue. There likely will be some....but unless its past .015" or more you probably won't see a problem as its duration is negligible. So far with a base gasket installed the Farmertec 56mm BB's I've used have been minimal or non existent. I had one that was tough to get to idle. Check that on those that failed as that's effectively an air leak if its large enough and has enough duration. OTHER brands I have checked have terrible free porting and I suspect would create an issue.
5) LAST but not least....when replacing a OEM top end on a saw.....did you solve the problem that killed the OEM top end? I've seen guys with a toasted saw drop a new top end on it and toast another one.....because it had other issues. Maybe case seals or intake tract issues.
The builds I have done with those 56mm's so far have been a lot of fun. Sleepers for certain. Hoping at some point Farmertec deburrs that sparkplug hole and raises the floor of the exhaust port just a bit, maybe .025"; maybe even more so I can first eliminate the potential for free porting and second give me a bit of lee way to cut the cylinder base and trim the squish band....as if those monsters need any more compression! So you vendors out there, you know who u are. Please make that request. Either way I'm going to be using those 56mm cylinders but a slight detail change can yeald a lot of rewards down stream for a hack like me. My two are running really strong. And before any one complains to much.. these top ends are like 40 bucks!
But to the question of have I tried the 390 parts yet....no. Looking forward too though as I have dead completely OEM ones on the work bench. Just got wore out in a logging operation.
Als0 if plug threads are leaking... a thread repair kit would add time and material cost, but would get that saw running
1) I use the OEM decomp, pulse line, and chain adjusters for a variety of reasons.
2) On some of the MS660 56mm BB cylinders I had to "deburr" where the spark plug hole intersects the combustion chamber. A little "flashing" sometimes is left in there and could break off.
3) On the 56mm BB's you need to check and see if the piston skirt interferes with the base gasket. You will feel it when your doing your squish test and rotate the piston through BDC without the piston ring....with the piston ring in as well I guess just I always do a check w/o ring to see how much squish the build has and also to be certain I have the clearance at BDC. I used a dremel tool or die grinder to open those gaskets just a little.
4) There is a potential for free porting on the exhaust on any brands 56mm BB kit for MS66o's , Farmertec is no exception. It's a geometry thing as a 56mm piston is not going into the cases. SO a shortened piston skirt is the answer and that potentially leads to a Freeport (exhaust and intake open to the crank case at the same time) issue. There likely will be some....but unless its past .015" or more you probably won't see a problem as its duration is negligible. So far with a base gasket installed the Farmertec 56mm BB's I've used have been minimal or non existent. I had one that was tough to get to idle. Check that on those that failed as that's effectively an air leak if its large enough and has enough duration. OTHER brands I have checked have terrible free porting and I suspect would create an issue.
5) LAST but not least....when replacing a OEM top end on a saw.....did you solve the problem that killed the OEM top end? I've seen guys with a toasted saw drop a new top end on it and toast another one.....because it had other issues. Maybe case seals or intake tract issues.
The builds I have done with those 56mm's so far have been a lot of fun. Sleepers for certain. Hoping at some point Farmertec deburrs that sparkplug hole and raises the floor of the exhaust port just a bit, maybe .025"; maybe even more so I can first eliminate the potential for free porting and second give me a bit of lee way to cut the cylinder base and trim the squish band....as if those monsters need any more compression! So you vendors out there, you know who u are. Please make that request. Either way I'm going to be using those 56mm cylinders but a slight detail change can yeald a lot of rewards down stream for a hack like me. My two are running really strong. And before any one complains to much.. these top ends are like 40 bucks!
But to the question of have I tried the 390 parts yet....no. Looking forward too though as I have dead completely OEM ones on the work bench. Just got wore out in a logging operation.
Als0 if plug threads are leaking... a thread repair kit would add time and material cost, but would get that saw running
