View single post by Jensenman
 Posted: 04-25-2015 01:58 pm
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Joined: 04-14-2005
Location: Columbia, South Carolina USA
Posts: 156
Valve stem excessive clearance will cause serious smoke this way: on deceleration in gear the throttles are closed which increases manifold vacuum. If there's any excess clearance at the valve stem, oil will get sucked between the stem and the guide. When the throttle is opened again, blue smoke.

On cast iron valve guides, the clearances need to be as close to minimum as possible. Bronze guides will need a bit more because they are prone to seizing. Since the 907 has cast iron guides and doesn't use valve stem seals in the first place, as Tim Engel says the stem to guide clearance should be minimal.