A reader asks: I’m doing the three main fluid changes in my 2005 Harley Fatboy. For the primary, I have used AMSOIL Synthetic ATF in the past. Is that a good choice?
Our answer: The use of automatic transmission fluid in Harley-Davidson primary chaincases has been a hotly debated issue for many years. Automotive ATF would not be our choice.
Why Do Some Use ATF In H-D Primary Chaincases?
This practice seems to have originated in the aftermarket clutch market and drag racing community. One of the main reasons is that the thinner ATF will allow the clutch to grab harder during aggressive riding.
Some aftermarket clutch-makers suggest ATF meeting the Dexron III spec, while others suggest Ford “Type F”.
We spoke to one aftermarket clutch-maker who suggested that Harley-Davidson Primary Fluid did not perform well with their Kevlar clutch plates. They claim that ATF is more compatible with their clutch plates (more on Kevlar clutch plates below).
What Is Our Oil Suggestion For Harley Primaries?
- Each is designed for primary chaincase service
- Provides maximum chain and sprocket protection
- Allows for consistent clutch performance
- Eases shift into neutral
- Will not thin-out under extreme heat
- Reduces friction
- Overbuilt for long service life
- Compatible with aftermarket Kevlar clutch plates
Is ATF Too Thin For Harley Primary Chaincases?
We submit that ATF is too thin for Harley primary use. It’s true that many Harley riders have had years of problem-free performance with ATF. We contend that a thicker oil will provide superior metal-to-metal wear protection. Chains and sprockets will last longer using a fluid with an appropriate viscosity.
Below are the kinematic viscosity numbers for AMSOIL Signature Series Multi-Vehicle Synthetic ATF, AMSOIL V-Twin Synthetic 20W-50 and AMSOIL V-Twin Synthetic Primary Fluid. The viscosity is measured at 100°C.
- ATF- 7.5 cST
- 20W-50- 18.5 cST
- Primary Fluid- 18.7 cST
Compared to 20W-50 motorcycle oil and the dedicated primary fluid, ATF is very thin. For reference purposes, ATF has a similar viscosity to 5W-20 motor oil.
The AMSOIL V-Twin Primary Fluid is an SAE 90. Some may wonder how the 20W-50 and 90-weight primary fluid have a similar viscosity. The reason is that gear oils and motor oils are given different viscosity numbers on the label to prevent confusion. But in terms of the actual viscosity measured in the lab, they are very close.
In conclusion, if you have a stock clutch, use a dedicated primary fluid or a suitable 20W-50. If you have an aftermarket clutch, we would still submit that a good synthetic primary fluid is the better choice.