Adding a decompression valve

[BiodieselNC]

I know you guys are going to get on me for this.....

So I have a Stihl FS250 strimmer that i modded a bit. Milling the cylinder yielded me about 175psi, but now I am having problems with breaking starter ropes and am wearing out the starter pawl. So I am wondering what it would take to add a decompression valve.

I know that the FS350 (and up) all come with decompression valves. Interesting though, that the FS350 has the same 40mm P/C as the 250 and it is rated at the same horsepower as well. So did they just drill a hole and tap threads? Does anyone have experience with this? Has anyone ever added a decomp to a cylinder that wasn't tapped for one?

I know this seems stupid to do for a 40cc motor, its not that I can't pull it over but more an issue of parts breakage.....

 

[ScottWojo]

Find a spot that you want it, drill the area smooth if there are fins. Then drill and tap. Use a Husky decomp valve. If you need a hole in your cylinder cover, make one too. If possible locate the decomp valve over the exhaust port.

Oh, remove the cylinder, of course.

Scott

 

[Axlerod74]

I once considered adding a decomp to a husqvarna chainsaw that had a boss for one and only needed drilling and tapping. My biggest condern was chipping the chrome lined bore when the bit broke through and thus wearing out the cylinder prematurely. Someone reccomended a high drill speed and using a small wood block inside the cylinder to drill into on the break through (like a soft backstop). This seemed like the best idea however, I decided the pull was not significant enough and did not install. Good luck.

 

[BiodieselNC]

No fins on top just on the sides....

Do you know what is the thread size and pitch that I would need to tap the hole for a decomp? And what size drill bit for it? (i would of course start out with a small hole and move up in size). As long as I position this up in the combustion chamber, I don't think i would have any problems with messing up the inside of the cylinder, right? Also, the spark plug is angled toward the front of the trimmer (with the exhaust off to the side) So would I put the hole opposite the spark plug, or still towards the exhaust (which would put it very close to the spark plug)???

 

[Dan_IN_MN]

I once considered adding a decomp to a husqvarna chainsaw that had a boss for one and only needed drilling and tapping. My biggest condern was chipping the chrome lined bore when the bit broke through and thus wearing out the cylinder prematurely. Someone reccomended a high drill speed and using a small wood block inside the cylinder to drill into on the break through (like a soft backstop). This seemed like the best idea however, I decided the pull was not significant enough and did not install. Good luck.

Isn't the crome on the cylinder wall? Wouldn't you be drilling a hole in the top of the cylinder where the piston should never touch?

OP....I'd buy the decomp valve, measure it, check thread pitch, buy tap and drill. Better yet, find a friend who owns a machine shop.

 

[ScottWojo]

Keeping my head cluttered with thread pitch and sizes for Decomp valves is not something I need to waste time and brain space with. When I do it, I buy the decomp valve, the tap and the drill. Once you have the Valve, take it to the place you get the tap and drill, and match it up.

And if you can put the decomp valve above the exhaust port in the area above the exhaust port. Or if you want and it would work better directly across from the sparking device. But my first choice would be the farthest point away from the intake port. I would try at all costs, to not cut into the side of the cylinder wall to add a valve.

If you are fearing this procedure, take the valve and the cylinder to a machine shop.
Scott.

 

[timberwolf]

I have put a number of decompressors in different cylinders. Common decompressors are 10mmx1mm thread.

As for the hole into the sidewall of the cylinder, it does not need to be as big as found on many cylinders, something about 3/32 is plently. No worry about the plating, just take a round diamond ball end burr a little larger than the hole and give it a little dressing after drilling to bevel the edges of the hole.

the tap Pilot hole for the decomp by the book is 9mm though I tend to go just a little tighter to get full thread depth and use an 11/32 drill bit. Letter "S" drill bit will work fine too given metric drill bits arn't that common in most shops.

Puting a decompressor in the top of the head can be a bit more difficult, measure a few times to get a good read on where the decomp will be on the inside, then drill a small pilot and check that it is in the right location. Clamp the jug back in the drill press using the drill inserted in the piolot hole to line it back up, then cut the seat, drill the actual pilot and run the tap all without moving the jug, this will ensure that the threads are aligned with the seat and the decompressor will seal.

A couple of cylinders I needed to build up a pad of weld to make a spot for the decompressor, worked fine but it is a one shot deal, too much heat and the cylinder becomes a couple hundred dollar paper weight.

Here is a 298 husky with a stihl 361 decompressor added to the top of the head.

 

[ncfarmboy]

I always add a decomp to all the 51's and 55 Ranchers I build they have the boss already on the cylinder. Done at least a dozen. I personally have never had a problem with the chrome chipping on the cyl. I don't have a head small enough (yet) to use small ball to debur the new hole I've just honed them with Flexhone. No problems so far. Am building 5100 Dolmar (it has a boss) and I'm going to add a Husky decomp.to it. Shep

 

[timberwolf]

One thing I have found is if a decomp designed to be used in the side of the cylinder is used in the top of the head they sometimes pop off too early, that said the stihl decomps may be better when mounting in the top of the cylinder head. Drilling out the vent hole on the side of the decompressor can solve the problem too and is usefull on bigger saws when pushing the compression above stock.

 

[simonUK]

Using a decomp of a Stihl TS400 may be the way to go. They are orgianally designed to be mounted at the top of the cylinder might work/might not. I know the Chainsaw king in oregon has shown a couple of old macs where they mounted decomps next to the spark plug so not impossible

 

[BiodieselNC]

Not to worry guys, I would never dream of drilling into the size of the cylinder wall, I would only consider a decomp up in the combustion chamber area where the piston will never be (and no Nicasil coating). I have a buddy who works at a machine shop so I will probably take some measurements and mark both the inside and outside of the cylinder to give him an idea where i want it. And it sounds like the consensus is that Stihl decomp valves may be better because they are typically found on the top of combustion chambers while husky's are on the side and pop too early.

Scott, do you mean keep it away from the transfers? I think there will be ample room to put it over by the exhaust......

 

[timberwolf]

I don't think there is any big concern in where it is put in the head, seen a few on the side of the combustion chamber, even number pro Stihls are in front of the plug, seen a couple with decomp in the squish band though I don't think that is the best practice. On race heads I have seen one,two and even three decompressors in all sorts of spots.

Big factor is going to be where it fits best and there is enough thickness to mount it. Ideallly the tip of the decompressor should be flush with the inside of the compustion chamber which takes a bit of measuring to get the seat drilled to proper depth.

 

[BiodieselNC]

Here's an update to an old thread. I finally added the decomp to the cylinder of my Stihl FS250. After some measurements, i drilled the hole, tapped threads, and did a little minor porting to the cylinder while i had it off. It was scary sending that first small drill bit into the head but after my pilot hole was lined up I felt more comfortable and it actually came out right where I wanted it to. Not too close to the spark plug but still far enough up in the combustion chamber to be away from the squishband. Being a 40mm bore, it was a really small area to work with.

While i had the cylinder off, i did a little port work. just basically tried to smooth out the intake and exhaust, and widen a bit. Exhaust there is a TON of room to add width and still be covered with the piston skirts. Intake, not so much room, as the ring locating pins are the limiting factor. So I didn't go too wide, trying to keep things balanced. Also, this engine is an open-port design, and i have no idea what, if anything, can be done to the transfers to help flow.

Also set the squish a little tighter at around .018" it was .040" with the base gasket, and about .023" running gasketless. I'm not sure if this is too tight but I figure with a 40mm piston and such a small squish band, this would be okay.

Here are some pics:

This shows fitment with the top cover/shroud. I cut the hole a little big so that I could still get my finger in there if I was wearing gloves. Not the best looking, I have thought about getting an FS350 cover as they have the decomp hole already
View attachment 183076

Here are some pics showing fitment on the cylinder. I believe the FS350 uses the same cylinder but comes factory fitted with a decomp valve. I think the 250's should come with them too....
View attachment 183077View attachment 183078View attachment 183079

Here is a pic inside the jug. I have not cleaned up from drilling/tapping but just to show fitment inside the combustion chamber
View attachment 183080

 

[BiodieselNC]

Pics added

So, here's another question. While I'm waiting for some Caber rings for the FS250, I'm wondering what kind of stuff can be done to this open port cylinder? Just intake and exhaust? Is there anything that can be done to the transfers?

 

[maxandme]

Adding decomp valve to 1989 Husky 61

Anyone got some specific info on this model? Pictures? Type and brand of decamp valve?

 

[woodeneye]

Going to attempt to plug this off a different way.

 

[sawfun]

Here's an update to an old thread. I finally added the decomp to the cylinder of my Stihl FS250. After some measurements, i drilled the hole, tapped threads, and did a little minor porting to the cylinder while i had it off. It was scary sending that first small drill bit into the head but after my pilot hole was lined up I felt more comfortable and it actually came out right where I wanted it to. Not too close to the spark plug but still far enough up in the combustion chamber to be away from the squishband. Being a 40mm bore, it was a really small area to work with.

While i had the cylinder off, i did a little port work. just basically tried to smooth out the intake and exhaust, and widen a bit. Exhaust there is a TON of room to add width and still be covered with the piston skirts. Intake, not so much room, as the ring locating pins are the limiting factor. So I didn't go too wide, trying to keep things balanced. Also, this engine is an open-port design, and i have no idea what, if anything, can be done to the transfers to help flow.

Also set the squish a little tighter at around .018" it was .040" with the base gasket, and about .023" running gasketless. I'm not sure if this is too tight but I figure with a 40mm piston and such a small squish band, this would be okay.

Here are some pics:

This shows fitment with the top cover/shroud. I cut the hole a little big so that I could still get my finger in there if I was wearing gloves. Not the best looking, I have thought about getting an FS350 cover as they have the decomp hole already
View attachment 183076

Here are some pics showing fitment on the cylinder. I believe the FS350 uses the same cylinder but comes factory fitted with a decomp valve. I think the 250's should come with them too....
View attachment 183077View attachment 183078View attachment 183079

Here is a pic inside the jug. I have not cleaned up from drilling/tapping but just to show fitment inside the combustion chamber
View attachment 183080

 

[sawfun]

I have thought about the same thing, why the 350 but not the 250. Even stock ot has stout recoil for a 40cc trimmer.

 

[Had Robinson]

Have you ever added a decomp valve to a cylinder head without an existing boss? If so, do you have suggestions or some photos? Thx

I always add a decomp to all the 51's and 55 Ranchers I build they have the boss already on the cylinder. Done at least a dozen. I personally have never had a problem with the chrome chipping on the cyl. I don't have a head small enough (yet) to use small ball to debur the new hole I've just honed them with Flexhone. No problems so far. Am building 5100 Dolmar (it has a boss) and I'm going to add a Husky decomp.to it. Shep

 

[Canyon Angler]

Wow, you want to add a decomp to a 40 cc motor? I have a 91 cc saw (660) that doesn't even have a decomp. (I've had nothing but problems with the decomp on my 361, so the hole on the 660 is plugged...and life is good.) Good luck, nonetheless...