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Discussion Starter · #1 · (Edited)
I pulled the motor to swap from a worn flat tappet to a new hyd roller and while it is out, I'm doing a little poking around.

Cylinders 2,4,6,8 have come in contact with the valves. 4/8 are the worst and 2/4 are slightly affected.
None of the pistons on the right bank have come in contact.
last time the motor was apart, I had the block 0 decked. I did clay the right bank but did not do the left in a rush to get it back together🤦‍♂️
I did not have an issue before the motor was decked.
The heads and valves seem to be ok to my untrained eye.
I filled up the chambers with alcohol and did not get anything passed the valves with the springs in. No makings on the valves that I see.
-20cc speed pro hyper domed pistons
781 heads with 2.19 and 1.88 valves
Comp cams 286H
1.7 roller tip rockers
Has anyone had any experience with somthing like this, and if so what was the culprit? Why would this only happen on one bank and do I need to replace the pistons?
 

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How long was the cam in there? Was the bad parts of the cam because of this piston bashing???

PS,, clay both banks after decking. No such thing as too much of a hurry to miss that lol. I ONLY clay 1 7 2 8 actually... Engine out you say? I'd replace pistons and bearings. But I'm a guy who don't like going back later on.. Take all advantage of the engine being out.

Check how much difference piston to deck clearance is.. is your valve guide worn? Check around. I bet something is off on the head. To make that big of a smash.. if your decking was off that bad You'd see it with a naked eyec...

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Exhaust valve would be the one to hit most likely do from cam or cam timing. Intake hitting would be most likely valve floating. Did you happen to measure the pistons depth in or out of the bore on all cylinders? (Verify zero deck) certainly the pistons are installed correctly. Your looks to be a intake valve relief.
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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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Are those pistons made for the bigger valves? I have an Isky piston valve notch tool if you want to use it pay for the round trip ride, it makes the radius larger and if needed deeper, mines a 2.30 but worked fine when I used 2.19/1.88 valves or just Dremell all the pistons the same for larger radius, weight & balance can get off if its not the same, I've Dremelled Harley pistons WO any problems
 

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Has anyone had any experience with something like this, and if so what was the culprit? Why would this only happen on one bank and do I need to replace the pistons?
The culprit is the fact that you dont have enough clearance :) Probably was close before you decked the engine and now you pushed it over the edge. How much cylinder wall clearance do you have ? The pistons are probably walking around a little bit too.

I did clay the right bank but did not do the left in a rush to get it back together🤦‍♂️
When you did that how much clearance did you have atr the edges ? I like to see at least .050"

The heads and valves seem to be ok to my untrained eye.
If the valves touched (which they obviously did) I would definitely have them pulled out and lightly refaced to be sure. You would be amazed at how a valve spring can pull a slightly bent valve closed.

Why would this only happen on one bank and do I need to replace the pistons?
Has the one head been milled more than the other ? Or the valve seats not cut in to the same depth ?

The pistons should be fine but I would pull all 8 and have them fly cut to give you enough room, especially if you are going with a larger cam. You need to be VERY careful having this done because it is very easy to cut into the top ring groove on those pistons.

Its all simple engine building 101 :) Check everything and then check it again.
 

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That has nothing to do with float. Its inadequate eyebrow clearance. The pistons have to come out and be cut. Like Bill said, I generally for .060. But what I do is pull a valve, and transfer punch the piston through the guide and scribe the valve diameter on the piston. And yes both sides of the block and different heads can locate the head in a different location in relation to the pistons.
 

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When you clayed the motor, how much clearance did you have? And did you neglect to check the radial clearance too? The areas on the pistons that were hit might be due in part to piston rock. Some piston rock is normal ofcourse, and that's why the radial valve clearance has to be checked also. If you just checked the depth clearance, that was where you went wrong, (aside from neglecting one cylinder bank ofcourse). I'm sorry to see this. Live and learn.
 

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Are those pistons made for the bigger valves? I have an Isky piston valve notch tool if you want to use it pay for the round trip ride, it makes the radius larger and if needed deeper, mines a 2.30 but worked fine when I used 2.19/1.88 valves or just Dremell all the pistons the same for larger radius, weight & balance can get off if its not the same, I've Dremelled Harley pistons WO any problems
Dremel is a wonderful piston machining tool.... I've Dremeled "Boost Ports" through the upper side of stock snowmobile pistons (have to remove and leave off the bottom piston ring) to use on my old race engines that used boost port cylinders.... those pistons were un-obtanium in '80s already, so had to resort to doing it the homemade way if you wanted to rock and roll.....
 

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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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This 454 still runs great just be careful around ring land, did this in 2016
 

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I don’t really think there is a cheap way out of this unless you like not sleeping well nights. Take the motor apart and re-do it correctly unless you wish to carry a basket in the trunk….just in case.
 

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ALSO, as bill K mentions in one of his last points, its likely you have two different CC chambers in each culinder bank. ( unless they have your block off badly) I have had to, not once have the machinist take unequal amounts off the head deck to get the two CC'd close. THis usually I beg him to set up the pipette to check. My GM o74s were WAy off, being decked so much. 3cc

SO cc a few chambers in each head. They could be as much as 2-3 cc off. I cant tell if the seats look deeper in one than the other in any of the pics. A bad valve job will do this.

which sort of points you right back to what vortech710 said. This is not a hand mixer, its a much more precise machine. With VERY specific clearances required for power and life.
 
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I am pretty sure I have used that piston in at least one Mercruiser engine and they all have the 2.19 valves. I dont remember having to cut anything.

Bottom line is you need to check everything and see what is going on. Especially with going to a roller cam.
 

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Judging by the pictures I would call that a collision not a kiss. Those are pretty deep impressions. You probably have some bent valves as Bill pointed out. The good news is it is all fixable. As mentioned start over and do it properly or you could end up with a pile of trash.
 

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If your good with a die grinder that could be fixed pretty easy, but the right tool is better choice. Hard to believe a 2.19 valve would do that, but I can see a 2.25 hitting in that spot.
 

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Discussion Starter · #17 ·
Just noticed this ? 2,4,6,8 is the right bank ??
I clayed the 1,3,5,7 sided. The opposite bank is where I had the interference, which I did not check 😫 The Pistons are the speed pro h118cp.
yea, that is the plan. It is definitely time to tear it down further and see what is going on.
I will probably get some forged pistons since I am using a little 174 blower as well.
 
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Not a builder, but I don't see how the guides in that bank can't be AFU as well. Valves were likely pushed hard into one side of the guide, I'd expect cracking or at the very least uneven wear.

Honestly, it looks like you've averted a total disaster - one valve head snapped off and it hits the fan very, very quickly.
 

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Man, thats a fact. A few thou away from a catastrophic failure.
 
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