Stihl 090 build

[ft. churchill]

I was my first time having to machine on a jug. Got 'er done in an hour flat including set up time. Damn aftermarket piston jug kit had .065 squish clearance with no gasket. There was no way around it the jug base had to be cut in order for me to give my client the best possible job on his Stihl 090. It's gotta be right or I'm not doing it.

 

[ft. churchill]

The four jaws are a pain to get in. I set up a dial indicator and find my run out across two of the jaws. Divide the runout in half and move the half distance towards the other jaw. Repeat at the other set of jaws 90* to the first set. It took about 30 minutes to get it to .0005 in each direction.

 

[ft. churchill]

Here it is before I started cutting. I think it's kinda risky as you could bust off a fin and have the whole thing come flying of of the lathe. I made small .005" cuts, didn't seem bad at all.

 

[mdavlee]

One way to do it. On a mandrel in the lathe I can usually get one in 5-10 minutes from setup to cut if I don't have to have a spacer for the live center.

 

[ft. churchill]

I wanted to do a mandrel, but all I had for stock was a 3" aluminum heavy wall pipe. No way to support the one end easily to make the cuts to get it down to 66mm.

Besides, I probably never do another one.

 

[TJ the Chainsaw Mechanic]

Subscribed!

 

[ft. churchill]

I just came in from test fitting the cut base jug for squish and timing numbers after the machine work. My mathematics were correct and I got .025" squish with a gasket. Much better than .065" squish with out the machine work.
Timing came out to 75* int
109* exh
121* trans

I'm going to shoot for 75* 100* 121* w/ 21* blowdown.

 

[ft. churchill]

Any one ever see one of these exhaust trumpets before? I've only seen one once, it was in a Craigslist add for a 090 hotsaw. Had a trumpet and a tuned expansion chamber with the saw.

 

[beaglebriar]

View attachment 3631 View attachment 3632

Any one ever see one of these exhaust trumpets before? I've only seen one once, it was in a Craigslist add for a 090 hotsaw. Had a trumpet and a tuned expansion chamber with the saw.

Cast stainless?

 

[ft. churchill]

Spent the day grinding on the ports and assembly. Mostly left everything alone other than moving the exhaust up to the proper height. No widening, fingers, or bridges. Ports were already very square. Just straightforward remove any casting imperfections.



Exhaust

 

[ft. churchill]

Cast stainless?

Cast aluminum.

 

[beaglebriar]

I bet it'll make your ears ring.

 

[ft. churchill]

So here's the intake side. Floor came out at 75* so just polished it up with 80 grit.

 

[ft. churchill]

I bet it'll make your ears ring.

It is loud as hell, both before and after the rebuild. It has more crack after rebuild.

 

[ft. churchill]

The one thing I did modify was the lower transfers. As this saw has slab sides on the piston, with no windows by the piston pin, the airflow has to go up a narrow slot alongside of the piston and the make a 90 degree bend to enter the lower transfer floor. I laid this floor angle back into the base of the piston. I removed as little as possible so as to not increase the crankcase volume, but changed the angle close to a 45 degree entry. The first pic is the stock lower transfer port.

 

[ft. churchill]

Alot of burrs and sharp edges on the ports as it came from the factory.

 

[ft. churchill]

Quite a bit of mismatch to the trumpet from the exhaust port. So i marked it with a scribe and ground it to line up perfectly

 

[ft. churchill]

Here's the trumpet port after grinding to match the exhaust port and a spark arrestor screen installed.

The reason for the screen is not spark suppression, but to keep debris and mud dabbers out.

 

[ft. churchill]

Let's talk clutches. This thing has the biggest, heaviest clutch I ever seen on a saw. It must weigh 5 pounds. It has a thick grooved asbestos linings on it, heavy steel construction, and I have never seen another clutch bearing with two rows of needle bearings.
Hey I had it apart, the bearing was dry, so I greased before assembly. I would be embarrassing for the client to burn up a crank with all of that new power because you did not bother to grease the bearing.

 

[ft. churchill]

I like how the trumpet looks instead of the muffler.