Question about heads.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
This piston had lots of nice valve imprints after it rotated.
The push rods also looked a bit worse after contact was made.
When milling the piston the valve reliefs being lower are not touched.
So after milling the bottom of the relief is closer to the crown height.
The push rods also looked a bit worse after contact was made.
When milling the piston the valve reliefs being lower are not touched.
So after milling the bottom of the relief is closer to the crown height.
Cargo Master
Joined: Nov 2003
Posts: 2,585
Likes: 1
From: Duluth, Mn.
Normally, if the head is surfaced. The valve stems need to be ground, to spec also.
If you drop the head .005 closer to the piston. Then the stems need to reflect the same .005. If not, they'll not only be .005 closer, but the push rod will open them .005 further.
If you drop the head .005 closer to the piston. Then the stems need to reflect the same .005. If not, they'll not only be .005 closer, but the push rod will open them .005 further.
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Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
Originally Posted by fonefiddy
Normally, if the head is surfaced. The valve stems need to be ground, to spec also.
If you drop the head .005 closer to the piston. Then the stems need to reflect the same .005. If not, they'll not only be .005 closer, but the push rod will open them .005 further.
If you drop the head .005 closer to the piston. Then the stems need to reflect the same .005. If not, they'll not only be .005 closer, but the push rod will open them .005 further.
Wouldn't the nature of the rocker arms put the extra travel at more like 10 thou? Doesn't really matter, I'll prolly get the offset pistons.
Cargo Master
Joined: Nov 2003
Posts: 2,585
Likes: 1
From: Duluth, Mn.
Yeah Dave, the rocker would put the lift over .005.
But every engine has different size rockers. So to get my point across, I just stated the obvious .005. You could measure them. But I believe most apps have been measured, and the machine shops know the specs for most engines.
But every engine has different size rockers. So to get my point across, I just stated the obvious .005. You could measure them. But I believe most apps have been measured, and the machine shops know the specs for most engines.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
Even not taking into consideration any mechanical leverage difference in the rocker design, .005 off the head puts the valve .010 closer to the piston before the valve stems are cut.
Take .005 off the head, .005 off the valve stem still moves the valve .005 closer to the piston because the valve seat is .005 closer to the piston.
The only thing taking the same amount off the head and valve stem does is makes sure the valve closes and the lift is not changed.
The only way to change the valve being closer to the piston when it is open is to reduce the valve lift or mill out the valve reliefs.
Changing the lift would be a counter productive.
Milling the valve reliefs would be a headache.
How I explained this is a bit foggy, but if you read it twice and think about it, you will see what I am trying to say.
Take .005 off the head, .005 off the valve stem still moves the valve .005 closer to the piston because the valve seat is .005 closer to the piston.
The only thing taking the same amount off the head and valve stem does is makes sure the valve closes and the lift is not changed.
The only way to change the valve being closer to the piston when it is open is to reduce the valve lift or mill out the valve reliefs.
Changing the lift would be a counter productive.
Milling the valve reliefs would be a headache.
How I explained this is a bit foggy, but if you read it twice and think about it, you will see what I am trying to say.
Last edited by Dave Sponaugle; Jan 26, 2008 at 05:20 PM.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
To cut the reliefs and the groove under the precup opening would take something more like a CNC machine.
The amount of material is not really the issue, to much material off the piston crown and the compression ratio gets low enough that it becomes hard to start.
.040 off my pistons dropped the compression ratio from 22.5 to 1 down to 20.25 to 1.
The lowest temp I have started it at without being plugged in was -7 degrees.
If I did not have manual glow plugs, it would not have started and continued to run.
For about the first 30 second the engine ran, I had to hit the glow plugs for 5 seconds every 10 seconds to keep enough heat in the cylinder for the fuel to ignite.
After it ran for 30 seconds or so, there was enough heat for it to run without help from the glow plugs.
The amount of material is not really the issue, to much material off the piston crown and the compression ratio gets low enough that it becomes hard to start.
.040 off my pistons dropped the compression ratio from 22.5 to 1 down to 20.25 to 1.
The lowest temp I have started it at without being plugged in was -7 degrees.
If I did not have manual glow plugs, it would not have started and continued to run.
For about the first 30 second the engine ran, I had to hit the glow plugs for 5 seconds every 10 seconds to keep enough heat in the cylinder for the fuel to ignite.
After it ran for 30 seconds or so, there was enough heat for it to run without help from the glow plugs.
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Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
Annother update.
I asked about getting the offset pistons for better valve clearence, and he explained to me that the overbored cylinders already have a shorter overall height to keep the same compression, so I guess theres no problem there afterall, since the engine will be getting an overbore....
I asked about getting the offset pistons for better valve clearence, and he explained to me that the overbored cylinders already have a shorter overall height to keep the same compression, so I guess theres no problem there afterall, since the engine will be getting an overbore....
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Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
Originally Posted by fonefiddy
David your post is confusing??
"he explained to me that the overbored cylinders already have a shorter overall height"
Widening a hole, doesn't make it any shorter?
Are the pistons shorter?
"he explained to me that the overbored cylinders already have a shorter overall height"
Widening a hole, doesn't make it any shorter?
Are the pistons shorter?
Senior User
Joined: Jan 2008
Posts: 224
Likes: 0
From: On a farm near Malmo
When I put my 466 engine together I had the camshaft custom ground. I used a old head gasket and torqued the head down. I put a dial indicator on the valve and measured the clearance between the valve and piston. The clearance was checked when the valves were on rockover, (both valves slightly open). I belive the closest point the valves got was around 10-12 degrees before and after top dead center. The piston actually catches up to the exhaust valve as it closes on the exhaust stroke, and the piston goes up. The intake valve then opens faster than the piston goes back down on the intake stroke. I had a local machine shop mill the valve reliefs deeper. To make it easier for them I had both reliefs cut to the same depth.
Laughing Gas
Joined: Aug 2003
Posts: 919
Likes: 4
From: Newberg Oregon
Originally Posted by David85
Yes, pistons are shorter.
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