My Sportster Page |
Here are some pics of the 1972 Sportster I built from a basket case. To my knowledge it is the only stroked Thunderhead equipped Sportster in existence. |
T TECHNICAL |
Ignition Crane single fire Clutch Barnett / Tamer II ick here to add your text. |
Engine Displacement 74 cubic inches Bore Stock Stroke 4 5/8" Compression 10.5 to 1 Heads Robison Thunderheads Pistons Arias forged Cams Andrews "X" Rods and Flywheels S&S Carb Mikuni 42mm Intake Custom made here to add Exhaust Supertrapp slip-ons |
Click here to add your text. |
Chassis Frame Widened for late model rear fender Shocks Progressive Suspension Fork springs Progressive Suspension Brakes Front: GMA Rear: Stock drum Gas tank King Sporty Paint Imron black Wheels: Front: 19" Borani Rear: Stock 16" |
THIS BIKE IS FOR SALE |
Facts Robison Thunderheads werecmanufactured briefly in the early 1980's.reThey were loosely based on the XR 750 style head,o and featured similar valve angles and combustion chamber. Being of a single carb design they would not flow as well as the XR head, but were a significant improvement over the factory Ironhead. According to dyno tests conducted by a popular magazine, they offered approximately 20 bolt-on horsepower. Some sources have estimated that as few as 300 sets were sold before the company went out of business for financial reasons. The Thunderhead kits included heads, rocker boxes, pushrods and pushrod tubes, and a small bushing to restrict the total advance of the ignition timing weights to 28 degrees. Rocker arms had to be sent to Robison to be modified, and a special reamer was provided on loan to clearance cylinder barrels for the pushrod tubes, which interfaced with the heads at a different angle from the stock units. Performance A published test inicated that a stock 1000cc Sportster equipped with Thunderheads was pretty well a neck and neck match for a Honda 750 4 of the same era. The present machine is stroked and otherwise modified with modern cams, exhaust, single-fire ignition, and a Mikuni 42. It is considerably quicker than the 1000cc model tested. I also own a 1996 1200 Sportster equipped with Lightning heads, Screaming Eagle cams, a Crane HI-4E ignition, and a Thunderheader exhaust. Subjectively, the Thunderhead equipped 72 is considerably quicker on the bottom end and in midrange roll-on acceleration. It easily does second gear throttle wheelies. The Evo seems to be quicker on the top end, and this is to be expected. The small valve / high velocity flow Thunderheads would be expected to have a strong bottom and midrange and to taper off at the top end, where the bigger valve Evo heads produce more power. I don't know which bike is quicker overall, but the 72 is considerably more enjoyable to ride. It feels much lighter and more responsive to rider input. The Evo Sportster engine is a more reliable package by far than an ironhead. No contest there. You can expect at best, 30 thousand miles on an ironhead before you will need new guides and a valve job, for example. It wouldn't surprise me to see an Evo engine go 100 thousand without being touched. There are lots of performance parts available for the Evo engine (see Zipper's web site for example), and not nearly as much for the ironheads. Thunderheads haven't been available since the 1970's, so if starting a project I would definately go with the Evo engine, unless I wanted to build a heritage type machine. Scott |