Nine pounds that make a difference for small motors

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Photo by: Paul Nasvik
The inertia ring is a 9 pound weight that installs on the back of the flywheel.
Photo by: Paul Nasvik
Here's a side view of it installed on the flywheel. The inertia ring is on top.

An unfortunate decision on trail left me with a shattered clutch disk. Once home, it's a simple repair, but like everything else, an opportunity for upgrade. I had previously determined that a high friction clutch isn't for me - not a lot of high revolutions in rock crawling - so I looked at other options.

I read about Jason Bunch's rock crawling Jeep on 4x4Wire's WRCC coverage. He used a cool trick to overcome the lack of horsepower in his four cylinder - a heavier flywheel. The flywheel's job is to keep the engine turning smoothly between strokes of the pistons. Jason's (and my) little four cylinder engine doesn't have a lot of power at the low end because (among other things) there's a lot of time between strokes. The engine slows down and can't recover. A heavier flywheel helps the engine maintain momentum - meaning it stays running at lower RPMs.

My TJ has pretty low gearing, but on steep faces, large rocks or other precarious places it stalls. In itself, stalling isn't necessarily a bad thing. What's bad is sliding back down because the engine doesn't have enough compression to hold the 4000-pound Jeep in place - the engine literally turns backwards all the way to the bottom of whatever I was climbing.

So I called Jason and talked about options. I ended up ordering a 9 pound Inertia Ring. The Ring bolts to the back of the flywheel and works like a heavier flywheel - I could keep my stock flywheel and not have to worry about the compatibility of a swap.


My installation notes are specific to the Inertia Ring obtained from Tri County Gear. I used the following tools:

To begin, remove the driveshafts, transfercase and transmission. Remove the clutch cover from the flywheel. Before removing the flywheel, I took some quick measurements to see if the ring would even fit behind the motor. It looked close enough to try, so I took the next steps.

Remove the flywheel and drill out the threads that secure the clutch cover. I then made a run to a machine shop and had it resurfaced.

Test-fit the ring to the flywheel, and the flywheel to the engine. In my case, I needed to remove some material from the engine block. To insure accuracy, I tightened the ring to the flywheel - didn't want to remove any more material than necessary.

Photo by: Paul Nasvik Photo by: Paul Nasvik Photo by: Paul Nasvik
I had to grind away parts of the engine block to clear the Inertia Ring. Here you can see how the ring fits in the "groove" created with the grinder. Notice the studs/nuts instead of bolts to hold the clutch cover on.

There were only a few areas that the Inertia Ring contacted the block, and the angle grinder took care of the problem. Jason recommended .030" of clearance at a minimum - more if the engine was older.

When I was sure of the clearance, I bolted the flywheel in place and installed the clutch. The clutch cover slides over the studs of the ring and nuts hold it in place (instead of the stock bolts).

Replace the transmission, transfercase, driveshafts, fluids, etc.

The final test was fitting the starter. As it turns out I needed to cut a large section out of the inspection plate and manipulate the rest of it with a hammer.


Photo by: Paul Nasvik
This hill used to cause the engine to stall...
Photo by: Paul Nasvik
...but no longer - straight to the top!

All the research I did said that this would be an amazing improvement - the engine would stay running even under tremendous load. The downside? The engine will have to work harder to accelerate. I figured that it couldn't hit freeway speeds much slower than it did, but I worried about it getting off the line (at a stop light).

The first drive was on the streets of San Francisco - great place to test out a clutch as there's a stop sign/light at every intersection and there are plenty of steep hills.

Everything worked like it should (a plus after that much disassembly) and the engine seemed to idle smoother than before. I couldn't tell much difference starting out, but it seemed to take a little less feathering of the clutch. Then again, I could have been feeling the new clutch.

I didn't notice any drawbacks (a major relief), but I did notice a quirk. Because the engine takes longer to slow down, I get a little bit of a surge when shifting. I've actually chirped my 36" TSLs in second gear!!

Since everything was working correctly, I went up to the Rubicon for some testing and relaxation - it was two months since I had shattered the clutch disk and I was eager to play.

On trail, it's everything I hoped for - the engine rarely stalls no matter the size of the obstacle. If I need to give it a little bit of gas, the engine responds immediately. If I didn't know any better, I wouldn't think I was driving a 4 cylinder.

I have greater confidence on the gnarlier obstacles than I used to, and I'm not needing the starter as often. All in all, a good thing.


Many companies make flywheels and inertia rings. Few make them for rock crawling, however, as there's more of a market for the race track. Many of the inertia rings and flywheels I researched were actually lighter than stock. To make matters worse, they were only available for common race motor applications (read: V8s).

A good machine shop should be able to make an inertia ring, but they'd need some good measurements. The nice thing about working with Jason was that he has already done this to his 4 cylinder competion Jeep.

If you have a 4 cylinder or another engine not commonly upgraded, I recommend contacting Tri County or your local machine shop. If you have an I6 or V8, chances are a race shop is familiar with other applications. In that case of some engines, there may even be stock applications that will swap in. Every engine is different though, so be sure to research the swap carefully.

It's worth the upgrade for a little more "ooomph" on the trail.

Contacts: Related Links:
  • Tri County Gear
    1143 W. 2nd Street
    Pomona, CA 91766
    Phone 909 623-3373
    Fax 909 469-4685

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