Read This Before You Buy an Underdrive Pulley

There is one thing on this page that catches people out, and it is worth two minutes now rather than a rebuild later.

An underdrive crank pulley replaces the factory harmonic balancer. It does not bolt to it — it takes its place. The factory item is a roughly 5 lb, 6" steel damper, and our billet pulleys are 0.5 to 1.5 lb at 4". That weight loss is the point, and it is also the catch: a harmonic balancer exists to damp crankshaft torsional vibration. Remove it and the crank is less protected.

For a race engine that is an accepted trade. For a daily driver, a tow rig or a crawler that sees long hours at low rpm, we would fit the factory replacement harmonic balancer, not an underdrive pulley. We sell both and we would rather you bought the right one.

Crank Pulleys — Choose by Groove Count

All three are CNC machined from American 6061-T6 billet, cut with lightening holes, and carry a laser-etched degree wheel on the face so you can find TDC accurately without a separate tool. Pick by how many accessories you drive:

Going to 4" from the factory 6" also slows every belt-driven accessory, which is where the second benefit comes from.

Water Pump Pulleys — and the One Most People Get Backwards

This is the section worth reading properly, because the "performance" choice is the wrong one for a lot of our customers.

At sustained high rpm an overdriven water pump can cavitate — the impeller spins fast enough to whip air into the coolant instead of moving it, and cooling gets worse rather than better. Slowing the pump with a larger pulley fixes that. That is what these are for:

6" billet, polished · 7" billet, black anodized, the largest reduction

But if the truck crawls, you want the opposite. At 1,500 rpm on a long climb the pump is already turning slowly, and underdriving it further is how you cook an engine at walking pace. For that the stock size 4 3/4" billet pulley is the right part — you get the billet look and the weight saving without slowing the water pump down at all. Same material, same machining, factory drive ratio.

High rpm on track: go bigger. Low rpm under load: stay stock size.

Complete kits

If you are doing the whole front of the engine at once, the underdrive pulley kit with the 7" water pump pulley pairs the crank and water pump pulleys so the ratios are right together, which is easier than assembling it part by part.

Belts change with the pulleys — the underdrive belts live on the cooling page with the rest of the accessory drive.

Stock Replacement and Hardware

Not everything here is a performance part. If you are simply putting the front of a tired engine back together:

The A/C and power steering idler pulley for 1985-1995 keeps constant tension in the accessory drive — a worn one is a common source of belt squeal and of pumps that never quite hold tension.

The harmonic balancer crankshaft pulley bolt is genuine Toyota 90205-14004, M14 x 1.50. It is a large-flange bolt that carries real load and it is not a place to reuse something tired.

There is also a genuine Toyota OEM outer crankshaft pulley for trucks with power steering, which bolts to the face of the factory harmonic balancer. Check first that your balancer has four threaded holes in the spokes — if it does, it will bolt straight up; if it does not, pair it with the replacement harmonic balancer. It is a stock part rather than a performance one, and it is also the right answer if you are adding power steering to a truck that did not come with it.

What We Would Fit

  • Daily driver or tow rig — factory replacement harmonic balancer, stock size water pump pulley. Leave the underdrive parts alone.
  • Crawler or large tyres — stock size 4 3/4" water pump pulley for the billet look without losing coolant flow at low rpm.
  • Circle track, drag or sustained high rpm — underdrive crank pulley matched to your belt count, plus the 6" or 7" water pump pulley to keep the pump out of cavitation.

Not sure which combination your build wants? Tell us what the truck does and we will spec it.

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