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Big Bore Stroker CR Mini
Big Bore Stroker KTM Mini
Big Bore Stroker KX Mini
Big Bore Stroker RM Mini
Big Bore Stroker YZ Mini
BB Stroker CR250 to 290
BB Stroker KX250 to 290
BB Stroker KTM250 to 290
BB Stroker RM250 to 290
BB Stroker YZ250 to 290
BB Stroker KTM300 to 340
BB Stroker KTM380 to 410
BB Stroker CR500 to 540
BB Stroker CR500 to 600
BB Stroker CR500 to 625
BB Stroker CR500 to 640
BB Stroker KX500 to 540
BBS Stroker KX500 to 600
BB Stroker KX500 to 625
BB Stroker KX500 to 640

Big Bore Stroker Build - KX640


KX500 to 640cc

year range 87-04

95.0mm bore x 90.0mm stroke

(actual size 637.2cc)


(This kit is installed by us and not available as a parts kit.)


Because of the Custom machining and special processes that are needed to build your engine to these custom build specs, allow a minimum of 14 weeks for completion after we recieve your engine and payment.

(This engine build will easily add 25-30% more Power)

Your complete engine is required for this custom build, your Carburetor is not needed because one comes with this kit. Your CDI box & exhaust will only be needed if you would like us to run your engine, and you do not chose to purchase a pipe option.


Running a FMF or Pro-Circuit pipe with the longer stroke will give your engine more Top end Rev then the stock stroke, but may be held back from it's full potential because of its belly and stinger size. The TSR Pipe specs supplied with this kit is designed for the rpm range of this engine build. Also the complete power valve is removed and replaced with Custom machined components that will allow the exhaust to flow freely at all RPM ranges. The PV is not needed with the Big Bore Engine.


Your 41mm Pro Series carburetor or your alternate choice, will be set close and should only require fine tunning for your altitude and/or riding conditions when you recieve it. Our 17 plate clutch kit will help to harness the HP and put it to the rear wheel.


A replacement parts list comes with this engine build for all custom parts used along with a spec sheet of your new bore and stroke, compression ratio, Carburetor jetting, ect. The parts selected for this kit are available from any dealer that sells these products, these custom parts are not modified after purchased, they are only labeled custom parts to this application.

Your engine will get a complete rebuild along with all the machining neccessary to build it. We will also machine & install a decompression valve for easy starting. The package price includes all parts listed below, labor and special machine work.



If you have any damaged transmission parts or any parts other then what is listed below, there will be an extra charge applied for the cost of the parts needed only, the labor for changing the parts is included in the package price.

Parts list: (total parts this kit $1,650.00)

  • Custom forged 95mm Wiseco piston kit
  • 21mm wrist pin & bearing
  • Custom oversize sleeve
  • Decompression Valve
  • Decompression Valve weld fitting
  • Custom Power Valve elimination components
  • New Custom Pro-X connecting rod kit
  • Cylinder Spacer
  • Crankshaft main bearings
  • Transmission ball bearings
  • Water pump rebuild kit (shaft, bearings & seals)
  • Racing Clutch basket
  • Complete performance 17 plate clutch kit (89-04)
  • Complete performance 15 plate clutch kit (87-88)
  • Complete engine seal kit
  • Complete engine gasket kit
  • 41mm Keihin pro-series carburetor
  • V-force 3 reed block assembly
  • TSR Designed Custom Performance Pipe Blue Print to build-it-yourself,

             The Print Includes Cone layouts, instructions & weld-on Flange with O-Rings

Labor and Custom work: (total labor $1,725.00)

  • Bore & sleeve cylinder for big bore piston
  • Stage II Porting & Head machining
  • Weld and machine crank wheels for new stroke
  • Balance and rebuild crankshaft with new connecting rod
  • Machine cases for stroker crank clearance
  • Machine cases for larger sleeve
  • Machine & weld cylinder for Decompression Valve fitting
  • Install new Racing basket on clutch gear
  • Clean engine and prepare for assembly
  • Install new bearings and seals
  • Rebuild engine complete with new Parts
  • Run engine on testing stand (optional)
  • Set up Jetting in Keihin or Optional Carburetor


Inquire for your Custom Engine plus Options Order Form


RPM-KX640BBS8704   $3,275.00

(Regular package price $3,375.00)

Custom builds are available - Inquire with your request's

Return shipping charge is free if you live in the USA

International return shipping, flat rate $280.00

Inquire with questions & availability joep@rpmmfg.com


Sleeves & Pistons are currently in stock!


The performance information listed below came from our porting software, and using the port area & port timing along with a few other variables simulates what the HP at the crankshaft could be in a perfect world. The HP number listed below is not the actual HP of this kit, and in no way are we suggesting that it is. We are claiming a very conservative 25-30% power increase, and until a Dyno report is available, this is all we are claiming.





Optional Carburetor Exchange

Lectron Tuned 44mm HV Racing Carburetor

(Regular Price $390.00) ---- Exchange Price $40.00

(exchange price is only available with kx625 or kx640 custom engine build)

Optional Add-on's

PVL Ignition with 17oz weight wheel $499.00

Powder Coating

Center Cases, Case Covers, Cylinder & Head $575.00

(price includes striping parts, mask & blast, mask & coat)

Custom hand built Pipe (with TSR software) $439.00

(Includes RPM Custom Silencer with 1.50 stinger mount)

FMF Pipe & factory 304 or shorty Silencer combo $329.00

Pro-circuit Pipe & Silencer combo $329.00

Replacement Big Bore Stroker Part's


95mm Wiseco Piston kit # 11094M09500  $197.00

95mm Wiseco Ring set # 9500HK  $40.00

21mm Wrist Pin # S568  $18.50

21mm Wrist Pin Bearing # B1081  $18.95

Cometic BB Gask kit (89-04 head, base & reed)  # RPM-KX5BBGK-8904  $40.00

Cometic BB Gask kit (87-88 head, base & reed)  # RPM-KX5BBGK-8788  $40.00

17 Plate Performance Clutch kit (89-04) # RPM-KX5CLU-17PL  $219.00

15 Plate Performance Clutch kit (87-88) # RPM-KX5CLU-15PL  $199.00

Custom RPM/Pro-X Connecting Rod kit # RPM-KX5CRK-BBS  $115.00

V-Force Replacement Reeds # 3P794A  $59.95

V-Force Reed gasket # G150  $9.95