Oil leaks and seals (NA, NB, NC)
An oiled-up back of the engine is one of the commonest Miata complaints, and the source is often cheaper than it looks: a valve cover gasket, a sensor O-ring or a cap. This page lists who sells the parts and what an owner thread said. It is not a repair guide.
How to read it. Seller claims are as read on each seller's own page on 2 October 2026. Owner accounts are from Miata.net forum threads, dated below, and are individual opinions, not advice. There are no prices on this page, and nothing here was bought or tested.
| Seller | What it says it offers | Fits (as stated) |
|---|---|---|
| Moss Miata, valve cover gasket by Mazda | a genuine Mazda gasket, OE reference BP05-10-235C; its related-parts list pairs it with a head gasket, RTV sealant and the water pump | 1994-2000 NA and NB |
| Moss Miata, camshaft position sensor O-ring by Mazda | described as the replacement crank angle sensor O-ring for NA Miatas, OE reference 9954-10-3801 | 1990-1997 NA |
| Moss Miata, rear main seal by Mazda | a genuine rear main seal, OE reference BP05-11-312 | 1990-2005 NA and NB |
| Moss Miata, NC gaskets and seals category | 14 items for the NC including a Mazda valve cover gasket (903-485), a Mazda rear main seal (903-465), a PCV valve and its seal, drain plug gaskets and a magnetic drain plug; I read the listing, not the product pages | 2006-2015 NC |
Moss's search results also show a Mazda front crankshaft seal for 1990-2005 and Flyin' Miata lists a rear crankshaft oil seal for the NA and NB and an engine rebuild kit for the NA 1.8; I did not open those pages.
What an owner thread said
- A 1999 with 128,000 miles (February 2022). An owner traced a large oil leak onto the transmission and the area behind the engine, and replaced the cam angle sensor cap with a new Mazda one. The two bolts at the back of the valve cover then would not line up. A reply explained that the NB used a different cam angle sensor but kept the NA-style mount and fitted a small cap to block it off, so the cap is held by two bolts and a flat metal band, and that RTV sealant goes at the corners where the cover's curve meets flat and at the two front areas by the cam gears. He suggested doing a new valve cover gasket at the same time to cure all of it. (Miata.net thread; first page.)
Search results for other Miata.net threads say the NA's crank angle sensor O-ring is a very common leak that can run to the bottom rear of the engine and look like a rear main seal, that oil at the spark plugs usually means a valve cover gasket, and that Mazda gaskets tend to seal better than aftermarket ones. I did not read those threads, so treat them as leads.
Not covered yet
- The NC and ND: only the NC part list is here, and no owner thread for either. A buyer's guide warns of sudden oil loss on the NC, which I did not trace.
- Oil pan and oil cooler leaks, power steering and differential leaks.
© 2026 Fischer Interactive LLC. Written for the Ref Sensor app's Miata users. The seller's page is the authority.