Re: Crankshaft flex and side covers
By: Lynn Cooley Date: July 8, 1998
We all are aware that a crankshaft will flex and bend. What we don’t know, or should I say didn’t know, is how much. We also didn’t know what kind of specific problems the flexing would create.
Obviously the .875" journal crank could flex more than the 1.000" journal crank but there is not as much difference as you might think.
As a lot of you already know, we have been conducting tests on our new .875" stroker rods. This has been ongoing for about 6 months and we've tried several engineering designs along with different alloys and finally arrived at the ultimate design and alloy. Now let the serious testing begin.
We have a dynamometer but felt that real testing is accomplished on our adult fitted Jr. Dragster and a custom built Kart.
Our first test engine was built using a Raptor block, .140" overbore, 4.225" ARC stroker rod with .875" bearing, 3.000" forged stroker crankshaft, ARC head, ARC flywheel and ARC billet side cover.
This engine assembly was carefully balanced by us.
Now, to be fair and honest, our goal from the get go was to break the rod. We really abused this engine. Our RPM limit was 9000, but we did hit 9300 a couple of times.
The first problem was a high RPM miss and then coil failure. The clearance between the flywheel and the coil was set at .020. When the flywheel rubs the coil the first thing that happens is an interruption of the magnetic field and the coil misfires. The next thing is that friction creates heat and fries the coil.
The flywheel was checked and was running true and the coil was not slipping on its mount. We finally had to set the air gap on the coil at .030 to keep it from contacting the flywheel. This seemed strange and, little did we know, our next lesson would teach us a lot.
About 10 more minutes on the Kart and we blew that sucker all to pieces. The only parts that survived were the carburetor and the ARC cylinder head. The ARC stroker rod looked like it had been through a war but survived - unbroken.
The crankshaft had broken straight down at the radius of the rod journal. This is exactly were it would and should break if the crankshaft bends and flexes. Incidentally this crankshaft is one of, if not the best on the market.
The first symptom of this flex problem was the high speed miss and then the coil failure. By examining where the flywheel had rubbed the coil the hardest, we determined that most of the flex occurs at BDC (bottom dead center). The second place was just after combustion or TDC (top dead center), both of which really make sense.
Think about it - cylinder pressure is at its highest just after combustion and when the piston reaches the bottom, there is no resistance to cushion the abrupt change in direction of the reciprocating weight. As the piston travels upward, it has compression to offer resistance in one stroke and exhaust gases to offer some shock absorption on the next.
On the same note, have you ever wondered why additional deck clearance is needed the more you increase engine RPM? Most engine builders think of rod stretch the same as you do in automotive.
In a Briggs application, crankshaft flex is the main problem. In fact, we checked every rod we had tested and didn't find any permanent stretch.
Our next test engine was built using a BlockZilla block, .174" overbore, 4.225" ARC stroker rod with .875" bearing, 3.000" forged stroker crankshaft, the new ARC BlockZilla head, ARC flywheel and a billet side cover (no name mentioned). The ARC side cover was still working its way through engineering.
With the air gap set at .030" between the coil and the flywheel, we ran the engine very hard for 10 minutes, stopped and immediately tore it down. The temperature of the crankshaft got our attention real fast. After 45 minutes the crank was still to hot to handle (have you ever bent a piece of wire back and forth and felt the temperature just before it broke?). We felt that if we had run it any longer, we would have broken another crankshaft.
One other problem that showed itself on the billet side cover (no name mentioned) was that the 0-ring seal had scuffed and chaffed itself almost into nonexistence along the top of the cover. Along the bottom and up each side seemed to be OK (remember, this was only a 10 minute run).
Ball bearings, by nature, have a certain degree of self-alignment built in. The inside dimension between the 2 ball bearings in the block is 3 ½ inches. It's not hard to flex the shaft .020" in one direction and still have a free spinning shaft. This translates to at least .040" total flex in the shaft and could probably go to as high as .050" and still free spin.
To add insult to injury, the manufacturer of the forged cranks we tested leaves entirely to much clearance on the slip fit bearing area on the shaft. This dimension should be .9995" not .998". This sloppy fit can allow another .015" flex and still free spin. Remember this lesson if you ever have to slip fit a bearing on a stock Briggs crankshaft.
To summarize all of this, it's no wonder the flywheel rubbed the coil and that the crankshaft broke.
Since the ARC BlockZilla side cover was still in the engineering stage, we added a second ball bearing to it and held them in place with 3 recessed button head screws. This created the first DUAL BEARING SIDE COVER.
After installing our new duplex bearing side cover on the same BlockZilla block with .018 clearance between the coil and the flywheel, we had no problems. The engine was run hard for 20 minutes then disassembled and with a shorter cool down period, the crankshaft was noticeably cooler. This was a huge improvement even though the crankshaft had the sloppy slip fit on the bearings.
One other plus over the (no name mentioned) side cover that we tested, is that the ARC BlockZilla side cover is truly light weight, and the ribbing supports on the outside of the cover and, more importantly, the outer perimeter ribbing reduce flex.
Our 0-ring seal showed just a slight sign of scuffing.
The testing was so successful, we decided to use this same technology on our Raptor side covers, and they will be available by the time you read this.
You should see a longer life of the Raptor blocks as well.
The primary reason for all of this testing was to see if our new stroker rod design would live, and IT DID. Both sizes are now in production and available.
See our "Product Showcase" for details of these and other new products.
We try very hard to keep our prices competitive, so look closely at what is furnished with this new ARC side cover design :)