The OS 91SX-H C-Spec
The new OS91SX-H C-Spec has been recently released in Japan and I now have one. I have some great pics showing the new counterbalance system, piston, crankcase, carb, etc. It's a completely new engine and with any luck will be everything the original 91 was supposed to be.
The new flat black Competition Spec 91 from OS Engines hosting a 3-needle 60K carb (Same carb as the OS70) and a stock gold anodized Hyperhead. It looks as different on the inside as it does on the outside.
Notice only one groove in the new lightweight piston, gone are the two oil grooves which came on the original 91SXH. The tall new gold Hyperhead is clearly seen in the two pictures above.
The above two shots clearly show the new counterbalance and notice the bearing visible behind the counterbalance on the C-Spec pictured left. Notice the counterbalance is no longer symmetrical but has an unusual offset with much more metal on the right side than the left.
Here's a shot showing the new conrod of the C-Spec. Notice groove in the center of the rod, this was done to lighten the conrod to improve balance to further reduce vibration.
The piston and piston pin are different as well, on the interest of improving balance and smoothnees OS wanted to revisit the piston as well as the conrod. I can see the pisiton has less skirting near the bottom than the stock 91 and lacks the dual "oil grooves". I chose not to pull the piston out of the liner for photos as I didn't want to disturb the new ring so I left it put for now.
In the photo above you can see the solid and much heavier conrod on the stock 91. The bronze bushing on the lower conrod looks to be about double the thickness on the new engine, You can see how thin the bushing is in this photo compared to the new one pictured above it. Also not the lower crank pin appears to be hollow on the C Spec instead of solid like this stock crank pin.
The two photos above show the slight differences in the crankcase shape, inside and out the new black crankcase is different. The ports have been altered for better performance. Look carefully at the area of the case under the exhaust port all the way down to the motor mount and you can see the shapes are different. The case has been "opened up" to allow for better scavenging.
Here is a top view of the new HyperHead and the old optional blue Hyperhead. They're bascially the same outer design, the new gold head has some additional machining on it either for looks or to reduce weight. I understand the squishband/combustion chamber is slightly different in the new head however I could see a difference. Minor coutour differences can easily escape detection to an average Joe like me...
Here is a photo of the new 60K carb mounted on the C-Spec, beside it is an older 60B I had on my original 91. The intake on the new carb is much larger, so large I can actually slide the 60B intake inside the 60K, yep... it's that much larger.
Here it is mounted in my Fury Extreme, I'm headed to the flying field tomorrow to get some fuel through the motor. I think it is going to be a winner. I will follow up with how it works out. I'm running WildCat 15% through the motor with a Muscle Pipe 2 and I'm NOT running a Perry Pump. I'll add one if I have to but I am hoping I won't need it on this engine.
With any luck I can get it dialed in pretty quickly and get a gallon or better through it tomorrow.
