Gx390 broken

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Dsx_fab

Member
0E1F3F07-558E-4E92-BBF3-6A52D8298C8B.jpeg



Well this doesn’t come as a huge surprise, being that I am running blue stripe valve springs on a stock rockers and rods. Yet I am surprised a little since, I assume this all happened at one time?
if it hadn’t, the bent pushrod (on the intake side) should have made it run like crap right? But today I broke my exhaust rocker, yet the push rod in there is fine and straight. I really doubt The piston contacted the intake valve, I don’t see how it could. Anyone have any insight for me?
This happened at about 5500-6000 rpms
 
Dsx_fab said:
View attachment 14190


Well this doesn’t come as a huge surprise, being that I am running blue stripe valve springs on a stock rockers and rods. Yet I am surprised a little since, I assume this all happened at one time?
if it hadn’t, the bent pushrod (on the intake side) should have made it run like crap right? But today I broke my exhaust rocker, yet the push rod in there is fine and straight. I really doubt The piston contacted the intake valve, I don’t see how it could. Anyone have any insight for me?
This happened at about 5500-6000 rpms
Click to expand...
A look at top of piston would confirm touch.

Most likely coil bind on intake spring.
May have been really close so lived for a while.
Bent pushrod may have helped it survive.

Exhaust same, but not quite as lucky.

Put it back together, check for minimum coil bind clearance at max lift with zero lash. Be sure to check retainer to guide clearance.
 
95 shaw said:
A look at top of piston would confirm touch.

Most likely coil bind on intake spring.
May have been really close so lived for a while.
Bent pushrod may have helped it survive.

Exhaust same, but not quite as lucky.

Put it back together, check for minimum coil bind clearance at max lift with zero lash. Be sure to check retainer to guide clearance.
Click to expand...
I have been running this engine for a while. I’ve checked for bind. I believe the exhaust side just happened to give up first but the broken rockers were inevitable. The bent rod however, I’m having trouble envisioning how that happened.
 
Really only two conclusions .
Either the pushrod was not strong enough to compress the spring .
There was contact some where .
 
flattop1 said:
Really only two conclusions .
Either the pushrod was not strong enough to compress the spring .
There was contact some where .
Click to expand...
Well I got it back together today, runs well with good compression. I was reading about backfires though.. I’ve never worked on pushrod engines but apparently it’s fairly well known that a backfire large enough can bend a pushrod. In fact, it backfired so bad that it slammed the choke shut and bent the butterfly! So who knows...
 
No any nick , sharp edge or mark in that area will cause a stress riser . Thats where the fracture starts . Of course it could be just not strong enough .
Hitting the stud is a possibility extending the slot , if its close would be the cure . Slip a peice of solder in up from the bottom rotate the engine . Then see how thick it is or if it shears it completely .
 
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