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Old 12-03-2012, 02:58 AM   #704
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Join Date: Jul 2006
Chapter/Region: SWIC
Location: AZ
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On this conversion, I only swapped the intake cams out with the USDM ones. Any time you swap cams, its probably good practice to check your bucket to camshaft clearances. Jay helped me do this. While we were in there, we decided to check the exhaust clearances as well, even though in theory, those should be fine from the factory.

This table shows the clearance specs (lower limit, nominal, and upper limit) and formulas prescribed by the FSM. I put this table together in Excel as part of a master speadsheet that automatically calculated clearances and selected the proper bucket size for a desired clearance.

For the initial measurement, we threw whatever buckets into whatever valve hole as a baseline. There are reflected in the left most columns under each heading. From there I calculated theoretical bucket size needed at a series of desired clearances. This was more academic than anything else.

This last table shows the calculated buckets needed to perfectly achieve the 0.008" intake and 0.010 exhaust clearances on each valve. Since the buckets are only available in whole numbers, I had the choice to select the next smallest or next largest whole number. Each increment (ex. 501 -> 502) in bucket size represents 0.01mm or 3 thousandths of an inch, so not much. In a few instances, I selected a bucket that was a bit larger or smaller than the most ideal size because it was readily available and I didn't want to have to buy a ton of buckets. A big thanks to Jay on this one for letting me raid his bucket collection.

Ultimately, you can see the bucket size I selected, and the resulting clearance. Then you can see my actual measured clearance for each valve. Not too bad!

Many shops/guys will run extra tight clearances to "add to" or "act as" a slightly larger cam profile, but this has its drawbacks. For one, your buckets tend to wear out faster and can split/break with time/wear. Another possibility is that as things expand with heating, a tight valve might not fully seat and give you lower compression in that cylinder. Lastly, your valves cool the most when they are shut (in contact with the head). If they stay open too much, they can be burnt with time. For these reasons, I decided not to go fancy and simply use the Subaru nominal values.

Here are the completed heads:

Lastly, Jay helped me install a new Gates timing belt kit. The stock components seemed brand new on this car (only 14K miles), but I figured this was as good a time as ever to draw a line in the sand and install a new setup.

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