Used Wrong Drain and Fill Washers for Differential
 

Hi all. Had my S2000 serviced at an independent Honda shop. One of the things they did was drain and refill the differential with Amsoil 75w-110. Looks like the copper washer (90402-PCZ-003) was used for the drain bolt, while the aluminum washer (90401-PCZ-003) was used for the fill plug. The washers are reversed. Is this going to be a problem? There do not appear to be any leaks.

Go easy on me...

I don't know the reason for the difference but I think you'll be fine, they perform the same function of being crush washers.

I have an opinion, but I don't know the exact reason Honda chose those particular washers:

Both washers are crush-type washers, so leakage shouldn't be a problem. Some manufacturers use copper for both and others reportedly use aluminum for both. In a severely corrosive environment the aluminum drain washer could corrode preferentially over the steel plug because aluminum is less noble in the galvanic series. Pitting corrosion occurs as an electrochemical reaction between dissimilar metals in a saline aqueous environment. It's sort of a makeshift battery that attacks the least noble of the two metals. Since the aluminum washer is replaced with each fill, a little corrosion is no problem.

Copper is more noble than aluminum in the galvanic series, so in a very corrosive environment the corrosion would occur to the aluminum diff housing where it touches the copper washer. But since the copper washer is extremely small in volume compared to the aluminum housing, any galvanic cell would be insignificant. In my opinion, the Honda engineers chose an aluminum drain washer over a copper one just to be certain they don't have issues in extreme environments.

The fill plug isn't a problem.

If the copper washer on the drain plug bothers you, I suggest you dab a little grease around the plug and call it a day.

I'm a Metallurgical Engineer and have a lot of experience with corrosion issues, which is where I am drawing this info from, not from any specific knowledge on diff washers.

The bolts have the same thread but different head sizes (23mm fill and 24mm drain) for obscure reasons. I changed my differential fluid two days ago and was thinking to myself that perhaps the different washer material is just there to help identify the bolts during maintenance. My procedure is remove the fill bolt, remove and replace the copper washer, and set aside. Same for the drain and the aluminum washer. Easy to identify the bolts when time to replace and torque.

Trying some AmsOil 75W-110 GL-5 synthetic hypoid gear oil in the differential. The bottles have a wide mouth and the little bottle pump won't screw on! :( Some of this now comes in squeeze bags. But the diff takes less than quart so it's not debilitating. This oil is clear, unlike the red tint of LE-1605 which has been in the car for a couple of years. Barely a smudge of color on the magnetic plug in the differential after about 8,000 miles over the past couple of years. Same for the Royal Purple SyncroMax in the gearbox with an aftermarket magnetic drain plug. Wanted to try Amsoil there too (because we all know AmsOil will even cure cancer) but the boutique dealer was out of town and then never responded. No local source for the Royal Purple but the dealer had MTF. It really doesn't seen to matter a bit as long as "spec" oil us used. Sent an engine oil sample to Blackstone.

-- Chuck

This is the reason. Maybe, the reason for that 3/8" female drive slot, for the transmission drain. Though, I believe thread diameter is different for the filler bolt. :ponder: Prolly, just a old-time convention thing.

Both differential bolts have the same torque spec (not surprising as they have the same thread). The torque spec on the gearbox fill and drain bolts is different which indicates their threads are different (but I've not tried swapping them). I have to use a crow-foot wrench fitting on my torque wrench for the differential fill bolt.

-- Chuck

Thank you for this excellent post! Real world engineers like you are a huge boon to these forums! I especially like your disclaimer citing the source of your logic. Please post more on these forums, kind Sir! Your logic is extremely sound.

I also always wonder how cost plays an effect on decisions that car manufacturers make. It would be interesting to get you to weigh in on this aspect as well, regarding the topic at hand. Are copper, or aluminum washers more expensive?

How 're-usable' are aluminum washers compared to copper?

Our engine, trans, and diff oil pans are made of aluminum. Does this mean that (by using aluminum washers for draining) corrosion is controlled to the interface between the washer and the drain plug (as it pertains to sealing surfaces)? Of course, a theory is a vector which has both a magnitude and directional component. If I understand you correctly, you can't comment on the magnitude of effect, but even getting your opinion on the direction would be great.

Anyway, I love detailed technical discussions like this and would love to pick/rack your brain over stuff like this. Even if you don't have a complete answer it could give us subjects to investigate.

Please confirm or refute anything I've said above :D

Thanks for the compliment. Copper is more expensive than aluminum so I assume copper washers cost more, although the cost difference is probably minimal. They're both crush washers, so technically they should both only be used once although many people re-use them without issue. The aluminum washers are softer, which is why they're wider than copper washers to allow each to compress appropriately when torqued down. But they both do a good job at sealing.

Galvanic corrosion occurs between dissimilar metals when wetted with water and some amount of salt. The greater the difference in nobility between the metals, the more concentrated the salt concentration and the larger the volume of each metal, the stronger the galvanic cell that causes accelerated corrosion. The steel bolts are more noble than the aluminum washer, so if the conditions exist to allow galvanic corrosion to take place the corrosion would occur to the least noble metal, which would be the aluminum washer.

Copper is more noble than steel, so if the conditions were right the corrosion would occur on the aluminum and steel surfaces. But because the size of the copper washer is so small, any galvanic corrosion would be negligible. Besides, all surfaces are wetted with oil and all bolts are coated (with Dacromet or zinc plated).

Google "Galvanic Series Chart" if you want to see the electrogalvanic potential between different metals.

I have a mnemonic device to remember which washer goes where. Duracell batteries are called copper tops, or used to be back in the day. The copper washer goes on the fill plug, in other words copper goes on top. Copper top, so I just think batteries and I never forget which goes where.

Copper top! Nice mnemonic. Been wanting to use that word all week. :)

There were copper washers on some drain or fill bolts of my olde Triumph 650 twin engine. I recall these needed to be annealed (heated red hot, then slowly cooled) or replaced during service. Annealing copper softens the material and in the case of a washer would allow a tighter seal. Despite this precaution I always parked the bike where the oil drips wouldn't stain the pavement.

-- Chuck