Re: Fire Crater pistons in M

Posted by Janicholson on January 24, 2006 at 07:09:23 from (199.17.6.175):

In Reply to: Re: Fire Crater pistons in M posted by MagMan on January 23, 2006 at 16:16:46:

Free lunch (almost)
Before putting things together permanently, try this:
Roll out a piece of kids modeling clay till it is 1/8th inch thick. Use a knife to cut out a oval to cover top of the #1 piston dome. Place the clay on the dome and stick it down a little.
Turn the engine till #1 is on compression TDC.
Put the gasket on the block (dry) and place the head on the gasket. Bolt the cylinder down snug ~10ft/lbs only on #1 cyl. Put the rocker assembly on and push both of the rockers down (one at a time) till either the valve spring is coil bound (coils touching each other) or the valve touches the piston (don't force it). Carefully and accurately (a depth gauge on a caliper is good) measure the spring height when the valve is all the way down, and when it is closed. The difference between these two will be the total travel till contact. If this measurement is greater than the valve lift specification for intake and exhaust, the valves will not touch the piston no matter what.
Gasket crush (for a composition) head gasket will reduce the travel .003-.010" but I think you will find plenty of room unless you are using high ratio pulling rockers and a high lift cam.
The marks in the clay show the location of the point of contact, and are proof of the fact that the valves touched down (or not).
The intake/exhaust TDC location (360 crank degrees from compression TDC) is the location for contact because the valves are (with some cams) both partially open at that point.

If the engine looses valve timing from broken cam, or cam drive, the valves will be in any position possible when the pistons are up. This worst case is the thing to avoid. If your engine has less lift spec than clearance as measured above, there is no way for the crash to happen.
Good luck, it is a bit complex,
JimN

Replies:

• Re: Fire Crater pistons in M Jason Simmerman 09:12:25 01/24/06 (0)

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