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Old 07-20-2008, 11:07 AM
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Starter gear to flexplate clearance

Before I order my new bolts (just got a new mini-starter), I want to make sure I am measuring the gap for the starter correctly. I used an 1/8" drill bit and could almost slide it between the flexplate gear and starter shaft (see attached). A shim or 2 should make it perfect. This is the correct place to measure the 1/8" gap, correct? It also looks like the 1 tooth is nicked on the flexplate as well.


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Old 07-20-2008, 11:46 AM
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Looks like you know what your doing, done your homework obviously.

The 1/8" clearance is a general rule and since you are measuring tooth tip clearance it is used only for a check.

If you manually move the pinion out to engage the ring gear and then "wiggle" the gear back and forth you should have 0.005" clearance minimum. Any tighter and it will bind. If you have no other way to check this clearance then a cut up feeler gauge will work.

The main issue is that this minimum clearance has to be in place on every tooth, normally I rotate the engine and check the clearance at 8 points on the ring gear to make sure there is clearance at every point of the ring gear.

You may be surprised to find the ring gear is not perfectly round and find a tight spot where there is no clearance and the pinion will not slide in easily, only then should you shim the starter for more clearance.

If I had a dollar for every SBC I've seen that had shims when none was required I would be rich, the factory mounting pads are usually right on the money and its the starter casting thats the culprit. Don't assume you need shims until you check, usually you don't.

P.S. Wouldn't worry about the chipped tooth, no big deal...this happens because a stone got thrown off the road and got lodged in the ring gear or some road debris hit the gear while it was rotating, excessive clearance at the pinion due to too many shims will chip the gear too.

Make sure you put the bellhousing ring gear shield back on when your done to prevent damage.
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Old 07-20-2008, 12:34 PM
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Thanks. I will check multiple teeth of the flexplate like you said.

I understand what you mean by pulling the pinion out and wiggling it to engage the ring gear, but not sure exactly what you are saying should be .005" min, between the starter gear tooth and the flexplate tooth?
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Old 07-20-2008, 01:10 PM
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I believe this is where you should be measuring

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Old 07-20-2008, 02:46 PM
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Dave K is correct. I've always used .032" (roughly the diameter of a paper clip). But I run Pontiacs and don't know about Chevys. The picture almost confused me because the starter is on the wrong side.
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Old 07-20-2008, 03:10 PM
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I have read about the paperclip between the 2 gears, but I recall several people mentioning the 1/8" between the starter gear shaft like this was a better way to shim it:

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Old 07-20-2008, 09:47 PM
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The paper clip method or 1/8" clearance is the laymans way of measuring gear thrust face clearance, ideally you want to measure the clearance at the point where the teeth contact. That is where the curved part of the tooth contacts at the pitch diameter.

Here is a picture of a gear to explain;



Where you are measuring the clearance or "backlash" is at the circular pitch line, I use a trimmed to fit feeler gauge for getting into the space. The reason why you don't measure any gear at the tips of the tooth is because machining tolerances can cut the pitch diameter on a different center than the OD diameter. If you have ever seen a gear with tips that seem thick on some teeth and thinner on others this is why, it doesn't affect the accuracy of the pitch line but it can fool you.

This is why I always recommend checking clearance with a feeler gauge on the pitch diameter not the bottom land or tooth tip which could be anywhere. This happens because the ring is machined on one machine but the teeth are cut on another, the tooth position or pitch diameter location is what is important not the OD center.

I know it sounds complicated but...cut a 0.005" feeler gauge to fit in between the teeth (about 3/16" wide is fine) and thrust the pinion out manually with a screwdriver to engage the ring gear, insert the feeler gauge into the clearance space and make sure it fits at 8 points around the ring gear.

If it does your good to go! If the clearance is a lot more (very likely) don't worry about it, you just want to verify there is enough. It will run with 0.080" of clearance and not be a problem, too tight is what you are checking for.
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Old 07-22-2008, 07:03 PM
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Cool. Thanks for the help.
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