Is it wrong to have to grind the piston for valve clearance?

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myasylum

myasylum

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I just question my shop sometimes, they know more than me, but at times, I'm like.... Are you sure about that?

This is one of those times.

I am getting a cast stroker engine, when when set the heads on the block to check clearance we noticed that the piston was hitting the head. We figured out that the casting where the intake valve is hangs down a bit to far.
He didn't want to screw up the valve job on the head, so instead he touched up a spot on the piston by grinding it down a bit. I thought, doesn't that than change the whole dynamic of the piston? Compression and all that. Does that weaken the piston? He just went along and did the same thing for all eight pistons. We put the gasket back on, and the head, used clay on the piston to make sure the clearance was correct, and it did'nt hit any longer so it worked? but at what expense?

I just thought I'd post a picture of the piston, and the dark spot is the area that he ground down a bit. What do you think of this method?

piston.JPG
 
I've ground, machined, filed and done lots of things to pistons to get clearance before, it shouldn't be a problem as long as you check before you do anything to make sure there is plenty of piston dome thickness left.
don't worry be happy!!!
 
Well the thing is too... the valves aren't even in yet! O.k, so it clears the head... but what about when the valves open and close??
 
Hi, is it the KB356 Piston? then it´s normal, they have to be machined because they stick out of the hole at normal deck heigt.

greetz Michael
 
I wonder why they don't clear as produced?
They ground all 8, to keep them all the same, and checked P/V clearance. I wouldn't want to grind too much, and weaken them.
 
Why use a set of pistons that need to be cut to fit???..something doesn't sound right..are these cheap pistons,i know the Diamonds in my 416 fit like a glove but thet aren't cheap either..
 
too many variables to make the call on this. i have reworked chambers to get clearance and seen pistons fly cut. were gaskets installed? what heads? how thick is the gasket compressed?
 
Over the years Chrysler has changes the deck height of it's engine several times. Piston manufacturers have to make the pistons different height for every one.
NOT, Keith Black has made one piston do the job of several and YOUR machinist can cut them to fit your block. Being a non-automotive machinist, it sounds like a good plan to me and a cheaper option for street oriented motors.
 
I've not heard of grinding the top of a piston except for valve relief. There will be some mfg deviations in the shape of the combustion chamber but I'm surprised they would be severe enough to cause interference. As long as the top of the piston is still thick enough and there are no sharp edges it should be okay. However, there must be some extra clearance for expansion rate differences. Was a head gasket used? I'm guessing that the thickness of a head gasket would be enough.
 
I am using J heads. Truth be told, I don't like them. I wish I had the money for different ones. My engine had two different year heads on it and I never knew that. One of the heads were from that late 70's the other is a 71 head. The 79 head was different than the other in a few different ways. I don't like my heads, I really don't! but this is what I have to work with. It's hard to explain but one head where the intake valve seat is sits on an angle it sits lower, and that is what was hitting the piston, the Exhaust was fine. The Gasket was .056 uncompressed. my machinist in his wisdom I guess felt that what he ground it down to was fine. He just made a groove for the part that was hitting, he didn't machine the whole piston. Like I say, just where the black dot is on the picture is where he ground down a bit. What about the Valves opening a closing? Might that be a issue too? I think the piston is a KB356 Piston. It is at the shop so I can't see it right now.
Thanks.
 
Here's what KB has to say about this piston:

"Centered pin. Use Magnum rod, small end narrowed to 1". Quench dome elevated .050. Machining require to achieve optimum quench. Closed chamber head requires additional clearance."


No, it's not entirely unusual to have to machine a piston, and it will not weaken it as long as it's done correctly. Having two different year head castings is not helping matters at all and is not a good idea. Have you checked the chamber volume on both heads? I bet one side is way different from the other.
 
Um... how sure are you the pistons are in properly? That quench dome... where you have to machine? It comes up under the wide open part of the chamber. Not near the valves... You should not have to touch the pistons unless the piston hits the large open area of the chamber. Can you get a pic of the pistons installed in the block?
 
Are you thinking that they are in backwards? It's happened before! He should be check valve to piston clearance too.
 
Oh and the CC's are different he said he's going to mill the head a bit to get them closer, but if they are slightly off it should be o.k.
Is that true?

I'm really not liking these heads, it seems like a mess, but it's all I can afford right now.
 
So basically the Quenched side should be on the spark plug side of the head?
maybe they are in backwards. Urgh?
 
360duster said:
Hi, is it the KB356 Piston? then it´s normal, they have to be machined because they stick out of the hole at normal deck height.)

If it is the kb356 piston it has to be milled to your spec to acheive proper quench for your set up. Those are what I have in my 408,they come with a extra tall quench pad.
Click to expand...
 
mod38-1 should know he researched this whole 408 thing long before any work was started!
 
Why would you pay good money for pistons,only to have to pay a machinist to cut them??...buy some quality ones from the start ie:diamond,ross....
 
myasylum said:
Oh and the CC's are different he said he's going to mill the head a bit to get them closer, but if they are slightly off it should be o.k.
Is that true?

I'm really not liking these heads, it seems like a mess, but it's all I can afford right now.
Click to expand...


DO NOT use those heads, you are just throwing good money towards bad. he may be able to mill one and have the CC's the same but that is not the only difference and the heads will be different heights. The heads are one of the most important parts when it comes to performance and if you take short cuts with the heads you will regret it later.

Depending on the heads and your machinist you can actually save money if you just go out and buy a pair of Edlebrock heads. You can just bolt them on and go, additional machine work is optional. To have a pair of used heads done right you can spend well over a grand.
 
I was looking at some other pics. I think I'm wrong there... I'm not sure how or why anything hits... but I have not used them before. Sorry about being a little fest in my response. As far as milling... if he means one head is larger than the other... that's normal. A head gasket job in th epast will do that. Milling to equalize doesnt hurt. So long as it is correctly milled this time, and check for the intake fitment during mockup... the intake flanges of the heads may need to be cut too.
 
I have these in my 410 and run j heads [profile gallery]

Here's the skinny, The quench pad comes up to the shallow side of the chamber, not near the plug, those are pop up 'dome' pistons.
The deal is you need to 1st figure what chamber volume/cc you need to achieve the compression desired, that means after port work, valves you need to get a barrette tube/plate kit from paw/jegs/summit and cc each chamber to find the largest chamber and then make the rest just as big, there are tricks like swapping around valves to attain that 1 cc-more common with used valves and stone VJ's btw.

Once you've evened them out and milled to the correct chamber volume for your app, you can now depth check the shallow side of the chamber or what some call the quench area of the chamber to find how much piston quench pad to mill off of corse figuring in the head gasket.
So say if you mill you're heads to 62cc and end up with .044 depth in the quench area, you need to mill .056 off the quench pads for a clearance of .039 w/ a .039 compressed head gasket.

Most important before milling is to get an 8 hole piston to deck height check to see if you really get all of that .100 quench pad out of the deck, and mark and keep all rod/bearings/pistons to their assigned/measured holes when reassembling after mock up and mill work.

Now the head issue, it wouldn't matter so much if they were both later heads like 76-79 but the latter heads have less short turn height and have bosses on the exhaust floors, so port matching them to flow alike would take a lil time.
Most 77 & up 360 smog heads ported go high 250's while old J's go 270-300cfm.

Beware, the chambers are considerably smaller than the bore and the quench pads might hit, so check it, if so open the head on that side but only as much as the cyl bore in relation to the gaskets bore.

Oh and yes the lil intake valve eyebrow comes pretty close to the pad.
 
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