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I recently rebuilt a 350 with flat top pistons (6.88 cc dish), vortec heads (906), and a comp cam. The block is an 1986 4 bolt main
Cam specs are
intake lift .447
exhaust lift .462
Comp xtreme 4x4 12-235-2

I also used a set of Alex's Parts valve springs that are supposed to increase safe valve lift to .550
Niether the block or heads were shaved
I'm using the pushrods from the '86 engine and the factory guided rockers that I got with the vortec heads

Went to start engine and pushrods bent.
Could I have adjusted the valves too tight or is the piston slapping the valves?
 

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The valve springs are binding. Use a feeler gauge and at full lift try to slide a .030 leaf between the coils.

.550 lift springs will not fit as a stock replacement item. The retainers and locks must be changed to allow at least .100" more installed height.
 

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Do the springs have inner coils? If so they will bind on the valve guide bosses (and you need to check for broken springs and bits). You can remove the inner springs and call it good, or have the guides machined down to accept the inner springs. The Vortec's guide boss is .845" (works with a single spring with an ID of .880"). Old Gen1 SBC heads' guide boss was .785" to allow for inner springs.

Your gross valve lift should be fine for unmodified Vortec heads...but you guessed it, you must verify that for yourself.

Are you using guide plates with your self aligning rockers? You cannot use both at the same time.

Read this: http://www.crankshaftcoalition.com/wiki/Vortec_L31_cylinder_head

And this: http://www.crankshaftcoalition.com/wiki/Valve_train_points_to_check
 

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There are four malfunctions that will bend pushrods.
1. Valve springs stacking solid.
2. Valves contacting the pistons.
3. Using stamped steel rocker arms with a stud slot that is too short, binding the rocker on the stud.
4. Bottom of retainer colliding with valve seal or top of guide.

I've seen it over and over on these forums where the builder will use the "twist the pushrod" method to determine when all the slack is out of the valvetrain and then add the preload. If you build motors all day long, every day, for a living, you will have the touch to do this. If not, you will get the valves too tight. The bulletproof way is to have a buddy hold the tip of the rocker down against the valve stem tip while you "jiggle" the pushrod UP AND DOWN with your thumb and forefinger and tighten the rocker nut with ratchet and socket in your other hand. When you can't jiggle the pushrod up or down any more, the slack has been removed and you can safely add the preload. You will want to use a very strong light to peer down the pushrod slot in the head to see that the end of the pushrod is in the middle of the lifter cup (in the depression that is made for the pushrod) and not up on the side of the cup against the lifter body.
 

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I doubt with the lift you have it is a valve to piston issue,I think it is a case of the wrong valve springs and possibly the rockers, long slot rockers should be used on any SBC with over .460" lift.Big mistake made by a lot of folks is mixing up parts:nono:,you should have gotten the matching Comp Cam valve springs they specified that's why they have engineers and dynos to coordinate parts to give you the buyer "more bang for the buck".
 
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