Two calls from 3.21.2015 show: Volvo Flame suppressor & Pressed-on Ring Gear?

  • Dorthea has a Volvo with an oil leak, possibly at the rear main seal. Ray said it might be a problem unique to Volvo’s called the “flame suppressor” or some term like that. What is the purpose of the flame suppressor on Volvos, and why do only Volvos have this part? And why would it cause an oil leak?

  • Kris has a 1983 Nissan Pickup w/ starting problems. Tom and Ray say the problem is in the ring gear on the flywheel. Ray says the ring gear is a pressed on part. I thought the gears on the flywheel were machined right into the flywheel itself. Is it common practice for manufacturers to press on the ring gear? If so, does that mean a defective ring gear can be replaced and the original flywheel kept?

A flame suppressor, also known as a flame arrestor, prevents a backfire from igniting volatile crankcase fumes, but I’ve never heard of one on a car engine. What we know as a Positive Crankcase Ventilation (PCV) valve is actually there for the purpose of preventing a backfire from igniting crankcase fumes. It’s a one-way “check valve” designed to allow crankcase fumes to pass into the engine’s intake while preventing flames from traveling in the other direction… from the intake to the crankcase.

Pressing on parts is common in industry. It’s called an “interference fit”, and usually is done by chilling the inside part to shrink it and heating the outside part to expand it (often at temperatures of 350F or thereabouts), then putting them together and letting them come to room temp. A 500 ton press, common in industry, will do the trick too, but is more likely to cause damage.

You can’t simply replace the ring gear on a pressed-on assembly. Once assembled, it’s pretty much impossible the heat only the ring and chill the center at the same time. It’s assembled for life.

@the same mountainbike

Is that like this guy.

Advance to 8 1/2 minutes into the video.

I’d think much of the grease would be destroyed.


That’s exactly the idea. And it’s an excellent example.
Freezing temps (typically 30F in a chest style freezer) should not hurt the grease. If it did, everyone in North Dakota and Minnesota… and Manitoba… would have serious problems.
And a regular oven won’t go high enough to anneal the drum, so he’s good there.

As long as his wife doesn’t find out. I can almost hear her now, “honey, why do my cookies taste like grease?”.

By the way, what he’s doing is not at all abnormal.

If the ring gear is an interference fit the truck has a flywheel which means it has a manual transmission.

If it were an automatic transmission, the ring gear would be skipped welded to the flex-plate.


European cars have flame arrestors and backfire suppressors while US cars have PCV valves. The European parts that I have dealt with had spring loaded diaphrams with a pintle valve attached. The part is notorious to fail on Volvos and cause oil to leak from several weak spots on the engine.

And the starter ring on a conventional flywheel is installed and removed by heating it. With an oxy-acetelene torch it is a quick short job once the flywheel is off.

But I’m just throwing in my 2c.

I thank you sincerely for your 2c. Your comments have taught me something I didn’t know before. A couple of things, actually. It’s a good way to end the day.

I was referring to the grease while the knuckle is cooling, @the same mountain bike.

I’d be afraid that the heat would transfer into the bearing and bake the grease into a tar like sludge. That little bit of water that he squirted onto the bearing would not compensate for the extreme heat held in the mass of the inner hub. Not that he should have hosed it down. He had the oven on the highest setting and by hosing it, I would be afraid it would crack the drum.


Flywheel ring gears can be easily removed by cutting through the ring gear almost to the flywheel with a friction cutter and then breaking the gear with a chisel. You replace it the same way the factory put it on. Heat it with a torch while the flywheel chills in the freezer, drop it over and quickly tap into place. Most people just replace the flywheel, though.

You do the same with the press-on ring used to hold axle bearing onto non-c-clip rearends like Dana axles.

Understood, Yosemite, but although the temperatures involved sound extreme to us, they’re not extreme to the materials involved. Heck, I used to work the flightline, on aluminum airplanes, in temperatures much lower than that in North Dakota.

Mustangman, if I were to do this I’d be more inclined to stick the ring gear in the oven. It’d expand it much more evenly.

Informative comments. Is it more economical to replace just the damaged ring gear, keeping the same flywheel, or do shops usually just replace the entire flywheel/ring gear ass’y when the ring gear is kaput?

Working with domestic light duty trucks it was standard procedure to replace the starter ring on the flywheel. The ring was cheap and easily replaced while a new flywheel was relatively expensive.

With a bolt or old screw driver clamped in a vice as an axis the flywheel could be spun while heating the ring with a torch and within 2-3 minutes it could be tapped loose and lifted off. Then the flywheel could be cooled with a wet rag while the new ring was being heated with the torch while hanging from the vice handle and in a matter of nimutes it would expand to begin slipping over the flywheel and easily tapped until fully seated. When replacing the clutch on trucks and finding signficant wear on the starter ring at the leading edge only I have often heated and flipped them to avoid a problem in the near future.

I don’t recall ever replacing the starter ring on a front wheel drive car. Most were automatics.

To remove a ring gear you use a chisel and break it. Then you can replace it. I had that done on my 65 Fairlane. Should have saved the flywheel and thrown the car away. It was a fun car but had no windshield washer, no backup lights and a one speed wiper system. Of course, there are other ways, see above.

TSM, I agree with the oven, IF the wife is out shopping, otherwise the torch will get the job done.

I found a couple of YouTube videos of techs doing the ring gear replacement with a torch for removal, too. Apparently not much heat transfers to the flywheel so heat the ring and tap it off. Reverse the procedure to install.

Point well made.
I had a wife once… {:open_mouth: