Establishing Proper Crank Hub Bore Diameter With ATI Performance

Establishing Proper Crank Hub Bore Diameter With ATI Performance

When it comes to fitting a damper on your crankshaft, anyone who has installed one knows it’s generally a tight fit. You probably know of someone who either “loosened it up a bit” so that it slid right on with no interference, or conversely, just ran it down with all of the ugga-duggas and called it good. Both of those methods are incorrect and potentially dangerous for your engine.

ATI Performance Super Dampers have a very specific (down to the ten-thousandth of an inch) tolerance range of interference (press-fit), which is actually a design feature of the damper assembly. Generally, the fitting of a damper is something that isn’t seen by the end-user, as any adjustment needed is usually performed by the machine shop or the engine builder. However, that leads to the incorrect thought that you can just run the hub down on the crank, or that you can open it up yourself to make it slide right on.

To correctly establish the proper interference fit of the hub to the crankshaft, the first thing you need to do is measure — both the crankshaft snout diameter, and the hub bore diameter. Generally, ATI’s hub bores come out of the factory with an inner diameter that will work with your crankshaft. But, sometimes, the crankshaft snout isn’t held to the same tolerances as ATI’s +/- 0.0001 inch.

If you measure and find that the crankshaft is slightly large, you have two options. You can buy a balancer, measure your crankshaft, and then take it to a machine shop to have the bore resized, or, you can press the easy button, and have ATI size the hub on your newly-purchased damper before they ship it out. It’s a $40 service, performed in-house at ATI, and takes all of the guesswork out of installing your damper.

The key to properly sizing your hub is properly measuring your crankshaft snout. This can be a difficult task if you aren’t comfortable running a micrometer to the fourth digit. To do that effectively, you need a micrometer that will measure that precisely — calipers and cheap mics just won’t cut it here.

That process simply consists of measuring your crankshaft snout with a micrometer and then furnishing those numbers to ATI. They will then send your damper out with a hub correctly sized for a simple bolt-on installation. “Using a micrometer is the hardest part of the process, because you have to get down to four decimal places,” says Harvey Baker of ATI Performance Products.

“A lot of guys will try and measure with calipers, but we need that fourth digit to get the fitment right. The hub usually only requires a very minor hone, but it’s the difference between getting it on all the way, and getting it stuck three-quarters of the way on and then having to pull it off and then sending it out to be honed. Really, if you somehow manage to get it on properly, even if it’s too tight, you’re OK.”

The problem is, that if the hub is too tight, you probably aren’t actually getting the damper to seat all the way. You might think it’s on properly, but it’s actually just the interference locking it into the incorrect spot. “If it’s too loose, you’re not going to get the protection the damper is designed to provide,” says Baker. “Plus you’ll be putting more load on the keyway than you should. That press-fit supports more of the load with the circumference of the hub than a factory damper will, reducing load on the keyway”

Now, I know a lot of you reading this are DIYer’s but Baker warns, honing the hub is not something that is an end-user task. “It’s not like you’re cleaning up the hone on an engine’s cylinder. It’s not as easy as that,” he warns. “You have to be careful when honing because there is a keyway in the hub’s bore. Really, it’s not something the end-user can do without a machine shop. If you look at all the zeroes in front of the tolerance, it’s an incredibly precise operation.”

ATI uses a Sunnen honing machine for its in-house hub-sizing service. If you measure your crank snout before you order your damper, they will deliver a perfectly sized assembly to your door. If not, you can always send your hub back to ATI for the sizing service. Either way, the service is only $40.

“Incredibly precise” isn’t hyperbole in this case. The target tolerances are generally plus-or-minus 0.0001-inch. Spelled out, that’s a total tolerance of two-ten-thousandths of an inch. On the smallest crankshaft snouts, you might pick up another ten-thousandth of an inch to play with, for a total of 0.0003 (+/- 0.00015 inch). When you really break it down, there is a reason that ATI offers the service for $40 or says that a machine shop needs to carry it out.

This snippet from the ATI Super Damper installation manual shows you just how tight the tolerance ranges are for proper installation. Also note, that while the instructions do say GM crankshafts are typically +/- 0.0001-inch from the factory, Baker warns that a physical measurement should still be taken. Trust, but verify here.

Regardless of who you have do the work, the key takeaways here are that the interference fit of the damper hub to the crankshaft snout is critical to the performance of the damper and the longevity of the crankshaft and that the tolerances for that fit are incredibly precise. It’s not only important that you measure all of the components to ensure the proper fit, but that you also achieve the proper tolerances to ensure the most performance out of your damper.

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