If you've spent any time under an old Chevy, you've probably noticed a drip coming from the th350 speedometer bullet area. It's one of those annoying little leaks that starts as a single red drop on the driveway and eventually turns into a greasy mess covering your transmission crossmember. While the Turbo 350 is a legendary gearbox known for being tough as nails, its speedometer housing—often called the "bullet"—is a classic weak point for fluid leaks.
Replacing or refreshing this part isn't exactly a weekend-ruining task, but it does require a bit of know-how regarding how the seals work and which size you actually need. Most people ignore it until their speedometer starts bouncing or the puddle under the car gets too big to ignore. If you're at that point, let's talk about what's actually going on inside that tailhousing.
Why is my th350 speedometer bullet leaking?
Usually, when someone says their "speedo is leaking," they're talking about one of two things: the outer O-ring or the inner seal. The th350 speedometer bullet sits in the tailhousing of the transmission and acts as a sleeve for the driven gear. Because it's located right where the fluid gets slung around, it relies on a tight seal to keep that Dexron inside where it belongs.
The outer O-ring is the most common culprit. Over time, heat cycles make that rubber hard and brittle. Once it loses its elasticity, fluid just weeps right past it. Then you have the inner seal, which is a tiny lip seal inside the bullet itself. This one prevents fluid from traveling up the speedometer cable. If you've ever pulled your dashboard apart and found transmission fluid inside your speedometer gauge, that inner seal is exactly what failed. It's a mess you definitely want to avoid.
Sometimes, the bullet itself is the problem. These housings are typically made of aluminum or plastic. The aluminum ones can get scored or pitted over decades of service, and the plastic ones can warp or crack if someone over-tightened the hold-down bolt. If the housing is distorted, no amount of new rubber is going to stop that leak for long.
Choosing the right size for your transmission
One thing that trips a lot of people up is that not all TH350 speedometer housings are the same. You generally run into two main sizes depending on the tooth count of your driven gear. There's a small diameter bullet and a large diameter one.
Most TH350 setups use the smaller 2-inch diameter th350 speedometer bullet. This size typically covers driven gears with tooth counts ranging from 34 to 39 teeth. If you're running a gear with 40 to 45 teeth, you'll likely need the larger housing. It's important to check this before you go ordering parts because they aren't interchangeable. The hole in the tailhousing is machined specifically for one or the other.
If you aren't sure which one you have, the easiest way to check is to pull the old one out. If it's about the diameter of a silver dollar, it's the large one; if it's more like the size of a thumb, it's the small one. Also, pay attention to the material. If you're currently running a plastic one and it's leaking, it might be worth upgrading to a billet aluminum version. They hold their shape much better and provide a more consistent surface for the O-ring to seal against.
Replacing the seals and the bullet itself
Swapping out a th350 speedometer bullet is a pretty straightforward job, but there are a few tricks to make sure it stays dry. First off, make sure the car is secure on jack stands. You don't need to drain the whole transmission, but expect a little bit of fluid to come out when you pull the bullet, so keep a drain pan handy.
You'll start by unscrewing the speedometer cable from the housing. Once that's out of the way, there's a small fork-shaped retainer clip held down by a single bolt (usually a 5/16" or 10mm head). Remove that bolt, pull the clip, and the bullet should slide right out. If it's been in there for thirty years, it might be stuck. A pair of pliers and a gentle twisting motion usually do the trick—just don't chew up the metal if you plan on reusing it.
When you have the bullet in your hand, take a look at the driven gear. Check the teeth for wear. If they look sharp or "hooked," now is the time to replace the gear too. If you're installing a brand-new th350 speedometer bullet, it usually comes with the inner seal already pressed in. If not, you'll need to tap that little seal in carefully. A small socket works great as a driver to make sure it goes in straight.
Before you slide the new assembly back into the transmission, lube up the outer O-ring with some clean transmission fluid or a bit of assembly lube. This prevents the O-ring from getting pinched or torn as it enters the tailhousing. Slide it in, put the retainer clip back on, and tighten the bolt. You don't need to crank that bolt down like a cylinder head; just snug it up so the clip holds the bullet firmly in place.
Calibrating your speedo while you're at it
Since you're already messing with the th350 speedometer bullet, it's the perfect time to fix that speedometer error you've been living with. If you've changed your rear-end gears or went to a taller tire size, your speedometer is probably lying to you.
The math is actually pretty simple. If your speedometer says you're going 60 but you're actually going 70, you need a gear with fewer teeth. If the speedo says 70 but you're only doing 60, you need more teeth. Each tooth usually accounts for about a 5% change in the reading.
Most people use a GPS app on their phone to get an accurate speed reading, then compare it to the dash. Once you know the percentage of error, you can swap the plastic driven gear that sits inside the bullet. They're color-coded based on the tooth count—like blue for 38, white for 36, or red for 37. Just make sure the gear matches the diameter of the bullet you're using.
Keeping things dry for the long haul
After you get the new th350 speedometer bullet installed, give the area a good cleaning with some brake cleaner. You want it bone dry so you can monitor it over the next few days. Drive the car, get the transmission up to operating temperature, and then check under there again. If it's dry, you're golden.
If you still see a leak, check the speedometer cable itself. Sometimes fluid gets wicked up into the cable housing if that inner seal was bad for a long time, and it can take a while for that old fluid to work its way out. Also, make sure the cable nut is snug. You shouldn't need a wrench for that; finger-tight is usually enough to keep the internal gasket compressed.
A lot of guys overlook this small part during a rebuild, but it's one of those things that makes a huge difference in the "cleanliness" of your build. Nobody likes a car that marks its territory everywhere it parks. Taking twenty minutes to swap out a worn-out th350 speedometer bullet is a cheap and easy way to keep your transmission fluid where it belongs and your garage floor clean. Plus, it's a lot nicer to look at a steady needle on the dash than one that's twitching because the gear is vibrating in a worn-out housing. It's the little things that make an old car feel "new" again, and a dry, accurate speedometer is definitely on that list.