Honda B/K Series Manual Transmission Mainshaft Thrust Shim

Honda B/K Series Manual Transmission Mainshaft Thrust Shim

So you're rebuilding your B or K series transmission with new synchros, bearings, or a new transmission case and you need to set the mainshaft thrust clearance? These are the OEM mainshaft thrust shims that control how much up-and-down movement (endplay) your mainshaft has. Part numbers range from 23291-P21-000 to 23986-P21-J00 depending on thickness. The shims come in thicknesses from 0.6mm (0.0236 inch) to 2.25mm (0.0886 inch). You won't know which one you need until your transmission's apart and you've measured the clearance with a dial gauge and mainshaft thrust tool. Honda's spec is 0.11mm-0.17mm (0.0043-0.0067 inch). If your clearance is outside that range, you need a different thickness shim or your transmission's going to have problems.

Here's Why Mainshaft Thrust Matters

The mainshaft's the shaft that holds your synchronized gears, 3rd, 4th, 5th, and on 6-speeds, 6th gear. The mainshaft has to spin freely inside the transmission case but it can't have excessive up-and-down movement. That movement's called mainshaft thrust or endplay. If there's too much thrust, the mainshaft's moving around and the gears aren't meshing cleanly with the countershaft gears. You're getting accelerated wear on the gear teeth and your synchros are taking a beating because the gears are hitting at weird angles. Eventually the trans starts grinding, popping out of gear, or synchros burn up. If there's not enough clearance - if the shim's too thick - the bearings bind up and create friction and heat. The mainshaft doesn't spin freely and you're wearing out bearings. The shim controls that thrust clearance and keeps the mainshaft positioned where it needs to be.

You Need to Check This When You Swap Parts

Stock rebuild where you're just replacing synchros and nothing else? You might be able to reuse the shim that came out. But here's the thing, transmission cases, bearings, synchro hubs, and distance collars can all vary by 0.05mm from Honda's spec. That's just manufacturing tolerance. When you start mixing parts - new clutch case, new bearings, aftermarket synchro hubs - those tiny variances add up and throw your mainshaft thrust out of spec. People rebuild transmissions with expensive Synchrotech or OEM Fresh synchros and don't check the mainshaft thrust. Six months later they're grinding gears or the trans feels tight and notchy because the thrust wasn't set right. If you're swapping the transmission case, replacing bearings, or installing aftermarket gear sets, you have to measure and set the mainshaft thrust. It's not optional.

How to Measure Which Shim You Need

You need a dial gauge and a mainshaft thrust tool. The thrust tool threads into the mainshaft and lets you pull it up while you measure the movement with the dial gauge. You can't do this accurately without the tool. People try using vice grips to pull on the mainshaft and it doesn't work - you can't get consistent measurements. Here's the process: assemble the mainshaft with the synchro hubs, distance collars, and bearing installed. Install it in the transmission case without the shim or oil guide plate. Bolt the case together, set up your dial gauge, thread the thrust tool into the mainshaft, and pull up while watching the dial gauge. Honda's spec is 0.11mm-0.17mm of movement. Your measurement's outside that range? You need a different thickness shim. Too much movement? Thicker shim. Not enough movement? Thinner shim. You're selecting the shim that gets your thrust clearance into Honda's spec range. It's a pain to measure but it's critical.

What You Get

• Honda OEM mainshaft thrust shim (individual shim, part number varies by thickness)
• Available in thicknesses from 0.6mm (0.0236 inch) to 2.25mm (0.0886 inch)
• Controls mainshaft thrust clearance (endplay)
• Installs in transmission case under mainshaft gear set
• Critical for proper gear alignment and bearing life

Fits These Transmissions

• 2013-2015 Acura ILX
• 1992-2001 Acura Integra
• 2002-2006 Acura RSX
• 2004-2014 Acura TSX
• 2003-2017 Honda Accord (K24)
• 2002-2015 Honda Civic Si
• 2001-2005 Honda Civic DX/EX/LX
• 1999-2000 Honda Civic Si
• 2002-2006 Honda CR-V
• 1994-1997 Honda Del Sol VTEC
• 2003-2010 Honda Element
• 2007-2013 Honda Fit
• 2016-2018 Honda HR-V
• 2000-2006 Honda Insight

Note: These shims are sold individually and you're ordering by thickness. You cannot know which thickness you need until you've measured your actual thrust clearance with the transmission apart and a mainshaft thrust tool. Don't guess. You can't return shims once you've opened them. If you're doing a serious transmission rebuild, buy multiple shim thicknesses so you've got options when you measure. Most transmission builders keep a full set of common thicknesses on hand. You need a mainshaft thrust tool to measure this accurately. Don't try to wing it with vice grips and eyeballing the dial gauge - you're going to get inconsistent measurements and pick the wrong shim. The thrust tool costs maybe $60-100 and it's worth every penny if you're building transmissions. When you're measuring, make sure the bearings are fully seated and the case is torqued together properly. If the bearings aren't pressed all the way in or the case bolts aren't tight, your measurement's wrong. After you've installed the shim and reassembled the transmission, some people pull it back apart and recheck the thrust clearance to verify it's in spec. That's up to you but it's good insurance. If your thrust clearance is way out of spec - like 0.05 inch when it should be 0.005 inch - something's wrong. Either you've got the wrong shim, the bearings aren't seated, or you've got mismatched parts that don't work together. Figure out what's wrong before you button up the trans and fill it with fluid. Wrong mainshaft thrust kills synchros fast and makes the transmission feel terrible to shift.