I noticed a small leak from the bypass valve nut on my FC and planned to replace the washer as a quick fix. I bought a washer from Aktins (Part #: 9956-41-800) but turned out to be too small. The bolt size is 29mm, but I haven't been able to find the right washer size. The Parts Rep gave me part number 995621800, but it turned out to be a drain plug gasket. Does anyone know the correct washer size and part number?

Here are some pictures (Aluminum washer is the part needed):

 

Mazda p/n 9956-42-200

https://mazdatrix.com/product/oil-co...mostat-washer/

 

Make sure you dont have a hairline crack on that. I had a small hairline crack on that opening and had to find a new oil cooler. I could probably have someone weld it but for now I have it for a spare.

 

I'll echo the above. I thought I had a leak too after a full drain of the pan and cooler. I replaced the washer again and it is still leaking. I've got a spare cooler ready to go, but haven't had the time to change it out yet.

 

Mazda washers are very simple, 99564-2200 for example
9956 = crush washer
4 = Aluminum, 2 is copper
2200 = Size of the ID in mm, so 22 = 22mm

9956-21-800 = a copper 18mm ID washer

Mazda has sizes from 6 or 8mm all the way into the 40's, makes it easy to measure your part, and come up with the correct number

 

Good to know!

 

I found that an impact wrench works best to remove the banjo bolts and thermostat bolt from the oil cooler. Sometimes, trying to wrench them off by hand will crack the aluminum cooler.