yamaha 250 piston in early 1980s husqvarna250

anyone ever install a yamaha yz 250 piston in a 82-83 cr 250 husky.i had heard that possibly pro circuit had been doing this in the early 1980s.dan

I have heard that also. I did some research on dimensions and they fit. I haven't compared piston ports though. I heard they have a little higher dome height= more compression. Mike Fitzgerald on the Yahoo Husky group has done it.

Yes, that was a pretty long thread. My main concern was where the locating pin(s) for the ring(s) were relative to the Husky ports. That's kind of a major deal.
The other item was modifying the piston crown for port timing.
Here you go, courtesy of Mike Fitzgerald:

Making a 1980-84 Husky 250 “Bark”


Port timing on the 80-84 250 motocross engines was set up to develop power in the upper 3/4 of the RPM range. They did that but just not enough of it. I believe the exhaust timing was 83 degrees ATDC (too soon) and the transfers opened 50+ degrees (too late) after the exhaust opened. The compression ratio was I believe 13.5 to 1. That translated into about 150 psi of
working pressure on top of the piston at TDC, which was OK, but the poor things got their butts kicked by the Japanese 250's.

A good running 2-stroke uses port timing in the area of about 85 degrees ATDC for the exhaust and about 130 degrees ATDC for the transfers. The 430's and 500's use this port timing. They both have a wonderful power bands. With a little tweaking (bump the compression a little) they both bark like bad dogs. BAD DOGS. The 500's had the huge grunt.

The way to achieve this on a 250 is to delay the opening of the exhaust port and get the transfers to open sooner, like at 130 degrees ATDC. I used a piston from a 78-79 Yamaha 250. It's a cast piece with two thin rings and can be fit a little tighter in the cylinder when boring. Cast pistons don't grow like the forged Mahle's when warmed up. I think they resist seizing better due to the higher silicon content. The distance from the wrist pin to the crown is about 4-6 mm higher. The wrist pin diameters are identical. This 4-6 mm difference is just the amount of height change to achieve the 85 degree exhaust port timing needed. An added benefit is the cylinder working pressure went up to about 175 psi.

The transfer port to exhaust port timing relationship is still wrong but the way to fix that is to reshape the crown of the piston where the transfer ports are. I did that by filing ramps in the crown of the piston at the transfer port position that would lower the crown, until I achieved the desired 130 degree transfer port opening. You can put the piston on the rod minus the rings but use the piston pin circlips to avoid having the wrist pin move sideways into the transfers. Use a Sharpie or a scribe to mark the edges of all four transfer ports on the crown of the piston. Match the angle of the transfer port passage walls on the crown. The crown of the piston has plenty of material to do this because the rings are set lower down the side of piston. There were a total of 4 ramps to match the transfers, two wide and two narrow.

The ramps naturally were at a steeper angle than the crown because the edge of the
piston is being lowered at the edge and go uphill toward the center of the piston. They didn't go all the way to the center; I didn't see the need, maybe 2/3 of the way to the center. There was no
modification at the exhaust port area because you want to maintain the exhaust port timing of 85 degrees ATDC. So when the piston was finished what it had in the crown was 4 ramps, all of them aligned with their own transfer port. A stock base gasket was used and naturally no head gasket.

The power band this configuration produced was torque and mid range like you've never
seen out of a 250 Husky. Trench digging, flip it over backwards climbing STEEP hills, pull hard from a dead stop in deep sand type of power. I let Dan Ashcraft ride my bike after the mods, he didn't come back for about 45 minutes. It was getting dark out in the desert and I thought he broke down somewhere or ran out of gas. When he got back, he said "Wow, this thing
pulls like a tractor. It just digs trenches." He did burn up all my gas.

I thought the power was a little too low in the RPM range so I added about .040 of gasket material under the stock base gasket, this calmed the bottom end a bit and moved the power up toward the mid RPM range. With the port timing relationship now where it should be, you could raise the exhaust port maybe 1 mm, use just the stock base gasket and get the thing to rev. The funny thing about this engine that I remember is that when I was climbing hills for the first time, it would make this loud, sort of nasal carburetor noise when it came into the meat of the power band. I thought the piston was going to come up through the gas tank, but it never did come apart. I think it had something to do
with the big air filter/air box arrangement they had on them. Stock reeds, close to stock jetting and
premium pump gas. That's all I can think of engine wise right now. The enduro bikes from those years used a different cylinder than the CR's. Their port timing was exactly what I had set my CR to but they didn't run like mine. Maybe the ports had different volumes. They weren't particularly fast. Pretty tame as I recall.

Mike Fitzgerald
Husky eGroup
Message # 6460

Thanks Peter!

I'm going to try one in my 430/250=300 project that Scooot got me wound up about!

Very intresting reading but I spot a few defects.

The reed cage changed in this 80-84 era on the 250. Not long ago it was stated elsewhere much more useable power improved I assume the reed cage change was what was the cause. Do those port degree numbrs really hold true across five years?

The mahle ( origional , stock ?)is a cast piston isn't it?

I will state I have seen a 250 husky lower end which plainly had yamaha in raised letters on the rod. This article makes me wonder if it was just a hack job or cost cut or was part of a planned engine build.

I could go on and on about tractors governors torque rise wheel scour which is way different than digging trenches.

I had issues/questions when I first read this post on Husky owners. Biggest question was how do you clear the head if the piston is 4-6mm taller from wrist pin to top of piston? Thicker base gaskets would further exacerbate the problem, a copper sealing ring would retard timing but 6mm would put the ring into the mating seal.
I plugged his numbers into my porting/simulation program and they didn't add up, the intial and modified port timing were not close to his description. Not saying it can't be done or won't work, but there's not enough info to make it work.
I own over a dozen Huskies with 5 or 6 running at the moment. The most fun to ride is my 83 250 xc with cut frame, 88 forks and revalved shocks. Other than it runs like a sick modern 125. Dirt rider dyno tests showed 33-35 hp all on top.
Over the years I have talked to George Erl and to Eric Gore about making competitive horsepower. They both said they couldn't make them run with thejapanese 250's of the era.
I added a v-force reed, tried the Husky bulletin mod of putting a wr sleeve in a xc or cr barrel and adding exhaust windows on each side of the exhaust port. I tried decking the barrel and reshaping the head. Nothing made a big difference. I thought about reshaping the port angles into the cylinder for better bottom end but lost interest.
After riding a friend's 79 250 Honda cr bored out to 300cc's ( approx.), I put some time into simulating a 300. With a 400/430 crank, it gives 297cc's. I was working on port height and spacer dimensions when I got side-tracked.

IF ANYONE CAN MAKE A 82-84 aircooled HUSKY RUN HARD, I'LLGLADLY SEND MONEY AND PARTS TO MAKE A VERSION FOR ME!!

In the meantime, I'll play with my 430's,500's and 510's.

And if anyone has a great running/riding 80's twin shocker, I'd love to ride/compare with you. I can probably get free ride time at Eddieville Motorsports park outside Goldendale,WA. to play with them.

Scott

You should have caught me and I would have given you the two-bit tour or let you try some of the old junk. I've got my 86 510 ready for our air-cooled class- 50mm wp conventionals w/ktm front disc, ktm40mm keihin fcr, rest is old Husky. Got the 83 250xc ready, trying to get my 83 500 cr back together( couple of gears were weak & a new piston and trying a decompressor ).

I'd swap you a ride on your new Husky and see how my old 2010 450 compares to it.

( I've heard more than once that the owner can be a J@**@$$)

Scott