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 Posted: Nov 15, 2019 10:13AM
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Article: link
Video: link

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: Nov 13, 2019 07:26PM
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Quote:
Originally Posted by Spank

…I guess you had  some rockers not backed off/ still set for piston position the engine was in and it was at a different point and when you tightened down the studs you opened a valve against a piston. 
We have a winner. I tightend the rockers down and was rocking the car back and forth when I discovered one of my valve springs was over-compressed. I backed off the stud and adjusted the valves. Fortunately, I did a compression test before I started the engine and discovered that the suspect cylinder had 0 psi. I took the head off and found a bent valve and push rod. 

I have trick valves with ground down stems which no doubt make them weaker down low on the stem. Stock valve on the left, my valve on the right. The piston was fine. Lesson learned.

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: Nov 13, 2019 04:43PM
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Yes, it is an interference engine.

I've had a valve hit a piston after valve float on a 998 with flat top pistons because it only used single springs on a 1275 head. No, it wasn't the exhaust valve hitting the block, it was the intake valve that hit the piston because valve float caused either a keeper to pop out or maybe it was the valve cap that broke-- tough to say because of the damage.  It not only bent the valve and broke the spring, it caused the piston skirt to shatter and scatter it's bits down into the transmission. When we pulled the head, we thought "maybe we can just replace the head because the valve indent in the top of the piston didn't look too bad". But nope, the shock broke the piston skirt.

I couldn't manage to watch the whole video but skipped through it and I guess you had  some rockers not backed off/ still set for piston position the engine was in and it was at a different point and when you tightened down the studs you opened a valve against a piston. 

Or a cam follower came up /out of its bore and the pushrod wasn't fully seated and you tightened head pushed open a valve while tightening...

 Posted: Nov 13, 2019 03:33PM
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A few weeks ago I asked if the A+ powerplant was an interference engine; would the valves contact the pistons if the timing chain broke? I had this happen on a V-8 MBZ motor once and it makes a big mess—heads, valves, rocker assembly, push rods, pistons and bottom end, all trashed. My question regarding the A+ was hypothetical. By an unfortunate coincidence, a few weeks later I had answered my own question. The answer is yes.

It wasn't the timing chain that let go, it was a head gasket. Thankfully the damage wasn't nearly as severe as with the MBZ. Watch the video and see if anyone can guess how a blown head gasket would lead to a valve hitting a piston.

https://youtu.be/Nz28jIrrses

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: Sep 2, 2019 07:49PM
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Quote:
Originally Posted by CooperTune
Depends on how you look at it. You could take some modeling clay and a some string roll the clay around the string making a worm about 1/8 in. dia. . With the engine cold remove the plug insert the worm toward the exhaust valve. Place in gear and roll it though several revolutions. If you lightly oil the the clay is should come back out and should indicate the clearance between valve and piston at the closest point. Steve (CTR)
Hmm… good idea. I could do that. Maybe I will. Thanks!

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: Sep 2, 2019 04:35PM
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Depends on how you look at it. You could take some modeling clay and a some string roll the clay around the string making a worm about 1/8 in. dia. . With the engine cold remove the plug insert the worm toward the exhaust valve. Place in gear and roll it though several revolutions. If you lightly oil the the clay is should come back out and should indicate the clearance between valve and piston at the closest point. Steve (CTR)

 Posted: Aug 27, 2019 07:40PM
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Quote:
Originally Posted by CooperTune
It would be hard to say without more information. I have never had a chain break or jump time. Valve float in a well prepared engine would be odd. Back in the mid 60s I think I recall floating valves in my stock 948 Sprite. Having vintage raced at 8700 and auto crossed minis to 9300 float has not been an issue. My 13 to 1 and above race engines have valve reliefs machined into the pistons. Most often the piston chasing the exhaust valve back to it's seat is area of concern. It's best to do the math and a clay check before finial assembly. Most engines this is not an issue. Steve (CTR)
Unfortunately, it's a little late for math and clay. But thanks Steve!

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: Aug 27, 2019 05:27AM
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It would be hard to say without more information. I have never had a chain break or jump time. Valve float in a well prepared engine would be odd. Back in the mid 60s I think I recall floating valves in my stock 948 Sprite. Having vintage raced at 8700 and auto crossed minis to 9300 float has not been an issue. My 13 to 1 and above race engines have valve reliefs machined into the pistons. Most often the piston chasing the exhaust valve back to it's seat is area of concern. It's best to do the math and a clay check before finial assembly. Most engines this is not an issue. Steve (CTR)

 Posted: Aug 26, 2019 09:58PM
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Q: Is the A+ motor an interference engine?  In other words, is it possible for the valves to come into contact with the pistons? I have 10:1 compression, a fast road cam, and 1.5 rockers. If my timing chain breaks or I experience valve float, will my valves hit the pistons?

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports