2016 July 26 Tech Metal & Shop
In another article we looked at fixing a bent crankshaft on a lawnmower. Removing the flywheel is part of that procedure (usually), but there are a couple important notes, so now those are here on this page.
Let's talk about removing a Briggs & Stratton flywheel. Most one-cylinder lawnmower engines are similar, so the general ideas should apply.
A Quick Note
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In This ArticleThe Right Way
Don't Have The Right Tools?
What Can Go Wrong
A Smart Idea
The Right Way
If you have the time and the patience, the right way is to order the flywheel puller for your engine.
This is the smartest way to remove the engine flywheel. You will greatly reduce the risk of breaking something.
Not every Briggs engine will use the same flywheel puller, but many of them do. Most of the Briggs & Stratton engines made for push mowers will use this one.
The only other tools you should need will be a set of wrenches, or sockets and a socket wrench. The genuine / OEM puller has self-tapping bolts; if your puller does not have self-tapping bolts, you'll also need a 1/4"-20 tap and a tap wrench for the holes.
No matter which method you use-- the "right way" or the other way-- you'll first have to remove the flywheel nut. An impact wrench works the best, but if you can wedge the mower blade temporarily with a piece of two-by-four lumber, you can use an ordinary socket wrench. Remove the flywheel nut and the cup that goes around it; then put the flywheel nut back on the end.
Don't Have The Right Tools?
The puller is not that much money, and you could probably have it within a few days of ordering. If for some reason you can't do this-- I don't know, you're out on the frontier or something-- then you might have to use "not quite the proper tools" for the job.
It's possible to hammer the flywheel off; the most immediate caution is not to damage the flywheel. It's made of aluminum.
Start with a rubber mallet. Basically you hit upward on the underside of the flywheel, hitting only the portions where the metal is thickest. (There are two of these areas, on opposite edges of the flywheel.) Alternate.
Rubber mallets are cheap; get two or three different sizes. Sometimes the flywheel can be loosened with a rubber mallet. You're not as likely to damage anything with that method, although I won't say it's impossible.
If the rubber mallet doesn't work, the next step is a metal hammer shielded with a thin block of wood. The wood prevents the metal hammer from deforming the edges of the flywheel.
For this method I've used a drilling hammer. This is basically a 3- or 4-pound ultra-compact sledge hammer. Be sure to alternate often; hit upward a few taps, then rotate the flywheel 180 degrees, then a few more, then rotate it again, etc. This method should slowly loosen a flywheel that can't be removed with a rubber mallet. It works because the heavy steel hammer transfers energy well, even when you don't have a lot of room to swing it. Don't forget to shield the impact with wood, or you will wreck the flywheel quickly. Now get yourself one of these drilling hammers and you will have one of the most useful tools in an entire metal shop.
Before you do this method, though, let's talk about...
What Can Go Wrong
The most likely thing to get wrecked would be the sump cover, also known as the bottom plate. This is where the engine bolts to the lawnmower. Hitting the flywheel with something heavy enough can transmit shock to the aluminum sump cover, which means it could crack. And that will probably start at one of the bolts.
I've been working on a mower that has cracks in the bottom plate, but it might be that the person hit a big rock or a stump. Any kind of severe shock to the engine (or the crankshaft) could crack something.
The good news: If you do crack the sump cover, used ones are not that expensive. Obviously there'd be the extra work replace it, though, so try not to crack anything.
Another possibility is to aluminum-braze the sump cover. You would have to remove the cover and de-grease it first, but I can't think of any reason why this wouldn't work. (I may end up having to do this procedure on mine.) These are special brazing rods that melt around 700 Fahrenheit, which is far enough below the melting point of aluminum that it shouldn't melt the engine housing / sump cover / etc.
And obviously the part to be brazed should be nowhere near any type of oil or lawnmower fuel.
Also, you may need the rubber mallet to separate the bottom plate of the engine from the rest of the engine housing.
A Smart Idea
OK, so let's say you're going to hammer the flywheel off. And obviously you don't want to crack anything.
Here's a smart idea, as long as you do this right.
Gently heat the flywheel with a propane torch. Start heating near the the outer edge of the flywheel. Going in circles, gradually move the torch inward toward the center, in a spiral motion. Don't heat the crankshaft. Don't heat the magnets on the outer edges of the flywheel. Don't overheat any one spot.
All you really need is for the metal to expand just enough that you can hammer the flywheel off with a rubber mallet, or a block of wood and a metal hammer.
It doesn't require much heat. 150 to 200 degrees Fahrenheit should do the trick. I wouldn't heat it more than that.
Removing a Briggs & Stratton flywheel is not that difficult, but it helps if you have the right tools. Even if you use the least-sophisticated method (hammering), there are ways to minimize the chance of wrecking something.
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