Back to Re-Do's Blogs
Jarret's 391 4-Mil Build - Completed June 2026

Jarret's 391 4-Mil Build - Completed June 2026

Jarret brought 2 Banshees into Re-Do Banshee's shop for engine work in mid-May 2026. This Banshee is Jarret's main ride - so he wanted to upgrade to a 4-mil stroker engine. Jarret bought upgraded parts for this soon-to-be 4-mil engine build. We recycled the stock crank and Pro Design Cool Head over onto Jarret's 364 Stocker Engine Rebuild. 

Once Virgil finished up Jarret's Stocker Engine, he got to work on Jarret's 4-Mil Build. This build was originally going to be a 379cc, but the machine shop said the cylinders needed bored out just a bit more. With its new bores and pistons, along with the +4-Mil crank, this engine's displacement comes in at a respectable 391cc. 

At 210 psi, this runs on race gas and really gets after it. 🔥

Watch the Build

Want to watch how this project came together?

  • 👀 Watch the video below. 
  • ⬇️Scroll down for the link to watch the project's whole playlist on Youtube. 
  • ⬇️Keep scrolling to see the Project Specs below the video.

 

Q- QUESTIONS About Your Project?
A- You can call 📞📱 or message us at 541-419-6054. 

Project Specs

  • BUILD NAME:  Jarret's 391 4-Mil Build
  • EXPECTED RIDING CONDITIONS:  Recreational dune & trail riding
  • DISPLACEMENT:  391 cc
  • ENGINE CASES (CASE HALVES):  Stock Cases
  • CYLINDERS:  Bored to 65.5 mm
  • PORTED:  Play Port by Redline Racing
  • PISTONS: Wiseco 65.5 mm - 795 Long Rod
  • CYLINDER HEAD:  Redline Racing Cool Head Anodized Black with RE-DO Logo
  • DOMES:  18 cc 
  • CRANKSHAFT: HotRods 58 mm long rod (4 mm over stock), trued and welded
  • COMPRESSION:   210 psi (helps determine fuel recommendation)
  • FUEL RECOMMENDATION:  Race Gas (110 Octane minimum to 116 Octane) for 150-200+ psi
  • TRANSMISSION:  Stock
  • REEDS / CAGE:  Stock - or - Moto Tassinari VForce4 Reed Valve System
  • CARBS:  Stock Mikuni
  • CARB JETTING:  380 mains 32.5 pilots 
  • NEEDLES:  5N7U needles for Stock Mikuni. CEL for Keihin PWK 35. 
  • NEEDLE CLIP SETTING:  Center Groove on Slide Needle
  • AIR SCREW ADJUSTMENT:  3/4 turns out
  • WATER PUMP IMPELLER:  Stock
  • FLYWHEEL:  Stock
  • CLUTCH BASKET:  Stock
  • CLUTCH:  Stock Clutch
  • CLUTCH LEVER:  Stock, ASV, or other
  • EXHAUST:  Toomey T5
  • STATOR:  Stock with Used Aftermarket Timing Plate, Cast Aluminum
  • TIMING:  Set to 4 degrees advanced. If detonation detected, change to 2 degrees advanced.
  • GEARING: 13/43 Tooth Sprockets? - Click for Info on Banshee Sprockets and Gearing
  • TIRES: 
  • SWINGARM: 
  • SUSPENSION: 

 

 

Back to blog

Do You Have a Project?

If you have a Banshee to restore, want to upgrade your suspension, or are planning an engine build, we're happy to help.

We specialize in Yamaha Banshees. If you have a different project in mind, contact us and we will let you know if we can take it on and what you can expect. 

CALL 📞📱 or message us at 541-419-6054. 

We offer a range of services. We can:

  1. Help you select parts for your DIY build.
  2. Help you Pack and Ship your Engine to us, build your engine, then ship it back to you to install.
  3. Complete the engine build for you and install in your Banshee.
  4. Inspect, repair and service the whole Banshee for you.
  5. Build or restore your Banshee.
  6. We may also be able to locate a local Banshee for you - then install your newly built engine in it.

Project References:

  • We calculate engine displacement based on the bore in the engine's cylinders and the stroke length of the crankshaft. Click here for more explanation in our Banshee Engine Displacement blog article. 
  • Click here for information about Banshee engine break-in procedures. This includes our recommendations for Fuel to Oil Ratios and Selecting the Correct Fuel.

About the Authors:

This project article was written by Janel with plenty of help from Virgil. Virgil is the Banshee mechanic, and I am the website mechanic. I do own and ride a Banshee, which helps. 

The Re-Do crew hopes sharing our project info and specs may help some with their own projects, or that you find it interesting. I want our technical info to be accurate and helpful.

Please do let us know if you like our project articles, or if you think I need to correct something. You can send an email to info@redo-parts.com - and your comments will be passed along to me. 

Thanks ~Janel 😉👌

Email to: info@redo-parts.com