RE:Flat Piston To Domed Piston - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-03-2007, 02:35 PM
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RE:Flat Piston To Domed Piston

Hello everyone, you were so much help with my last question that I thought I would give you another one. Here it is. I have flat head pistons in my engine now and I want to go to a Domed head piston for a little more compression. How do I measure so that I know I have enough clearance for the domed pistons to fit? I know I need clearance for the heads, just don't know how to go about the figureing part of it. I believe I'm running a 7 in. rod now, would I have to go to a shorter rod? Say 6.75? Your help is always appreciated. Thanks...Dave

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Old 09-03-2007, 03:13 PM
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Hi Dave. I went back through your posts from the beginning to see if I could get an idea of where you are on this motor. Saw where you did a cam change. What are the specs on the cam?

I don't recommend domed pistons at all, preferring to use heads with smaller chambers and flat-tops for better flame propogation across the chamber. Domes tend to get in the way of the flame front.

At any rate, you don't want to change anything until you know where you are with the combination of parts you have now. If you would answer the following questions, the fellows on the board will have a clearer understanding and be able to give you proper guidance.

Percentage of street use and race use with the car.
Car weight.
Engine size (I know it's a 350, but what's the bore?)
Rear differential type and gear ratio.
Transmission type.
Torque converter stall.
Heads manufacturer and part number.
Valve sizes and intake/exhaust runner cc's.
Intake manifold type, manufacturer and part number.
Carb type and size.
Piston deck height, piston manufacturer and part number.
Head gasket thickness, manufacturer and part number.

By the way, there is no way to get 7" rods in a 350 Chevy. You may be thinking of 5.7" rods.
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Old 09-03-2007, 04:02 PM
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like tech said he knows it a 350 but can you bring EVERYONE up to date on all the engine specs. thanks
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Old 09-03-2007, 05:05 PM
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Flat piston

Usually it's done by putting some putty on top of the piston, crank the engine over by hand with no spark plugs in it and STOP if you feel any resistance. If you can turn the engine over take it apart when you're done doing a minimum of 2 complete revolutions and measure the thickness of the putty. Off the top of my head I don't know what the minimum thickness should be.
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Old 09-03-2007, 10:26 PM
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I like to see 0.125" on intake and exhaust, but might settle for 0.100" on the intake depending on revs, cam, springs, etc.
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Old 09-04-2007, 06:04 AM
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Quote:
Originally Posted by ernkazern
Usually it's done by putting some putty on top of the piston, crank the engine over by hand with no spark plugs in it and STOP if you feel any resistance. If you can turn the engine over take it apart when you're done doing a minimum of 2 complete revolutions and measure the thickness of the putty. Off the top of my head I don't know what the minimum thickness should be.
A safer, albeit more expensive, method is to use light springs on the valves for checking purposes. With the rocker arms removed, bring the piston to TDC. With a dial indicator on the valve stem, push the valve down until it contact the piston. Record the distance traveled. If it is less than the lift times the rocker arm ratio plus the desired minimum clearance, you should cut reliefs until you do have the proper clearance. This method usually makes bigger valve reliefs than the putty method. However, it assures the pistons will not contact the valves if the timing chain fails. Check out this kit. http://www.jegs.com/webapp/wcs/store...10002_33102_-1
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Old 09-04-2007, 10:19 AM
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The checking springs are a good method for seeing your piston to valve clearance. But be sure to check the clearance through the whole revolution of the cam. If you just check it at TDC your reading won't be giving you the full story. You may run into tighter clearances during the intake and exhaust stroke. I know with my 383, during my intake stroke is where I get the closest distance between my piston and valve.
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Old 09-04-2007, 11:46 AM
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Huh?

As far as I know the putty on top of the pistons has nothing to do with cutting valve reliefs in them. You measure the thickness of the putty to know what your valve clearance is. What does a degree wheel kit have to do with valve to piston clearance? Are you suggesting you should retard or advance the cam in relation to crank rotation to gain clearance?
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Old 09-04-2007, 12:19 PM
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Ps

I can see how the method you describe will work, but rather than buying weak springs you could just use tape or a rubber band around the valve stem to hold it up until You're ready to drop it on top of the piston to take your measurement. I could be wrong about this, but I thought the camshaft lift spec included the multiplication ratio of the rocker arm. For example a cam with .500 lift on a SBC assumes a 1.5 ratio rocker arm. I'm pretty sure the afore mentioned cam isn't lifting the valve 3/4 of an inch off it's seat. I could be wrong about that though.
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Old 09-04-2007, 12:34 PM
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I concur w/ techinspector, domed pistons have no place in most engines and certainly not in any street engine. Unless there's a need for an extreme C/R, say beyond 14:1 there should be no need for domes to achieve any C/R within reason. And besides for the reasons he stated, they are also much more prone to detonation.
As in an example, the built LT1 in my daily driver has an 11:1 static compression ratio. This was achieved with a -16cc dish, or as SRP calls this particular piston, inverted dome. I have 56cc chambers and the pistons are, for all practical purposes, at zero deck with an .039" gasket.

Quote:
Originally Posted by redsdad
A safer, albeit more expensive, method is to use light springs on the valves for checking purposes. With the rocker arms removed, bring the piston to TDC. With a dial indicator on the valve stem, push the valve down until it contact the piston. Record the distance traveled. If it is less than the lift times the rocker arm ratio plus the desired minimum clearance, you should cut reliefs until you do have the proper clearance. This method usually makes bigger valve reliefs than the putty method. However, it assures the pistons will not contact the valves if the timing chain fails. Check out this kit.
Mininum valve to piston clearance does not occur at TDC. It must be checked throughout a full cycle for a cylinder, which is two crank revolutions. I think perhaps you're confusing this with degreeing a cam. However, valve (determined by cam) timing does indeed affect piston to valve clearance. This is why it must be checked after final cam timing has been determined and set. To check piston to valve clearance on an engine with hydraulic lifters, a pair of lightweight "test" springs will be required so as not to depress the lifters during the check. These can be purchased or made. I wound my own from tempered wire. Only enough spring pressure to hold the valve up is required, a very few pounds at most.
Also in most engines, even adequate piston to valve clearance will not prevent contact if the cam were to go out of time for any reason, such as in the case of a timing chain failure as you mentioned. Most engines are "interference" engines. This means that the valves and pistons occupy the same space but at different times. If the cam goes out of time for whatever reason, contact and some resulting damage is all but a certainty.
Pushing the limits, you can run a minimum of .080" intake and .100" exhaust. But be very sure of all of your measurements if you need to squeeze things this tight for some reason. Generally, with the right combination of parts in all but a rather radical engine, you should end up with some clearance to spare. But always check it. Not doing so can result in considerable lightening of the wallet! And I know this isn't where you're looking to save weight.

Last edited by Notorious; 09-04-2007 at 12:54 PM.
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Old 09-04-2007, 12:59 PM
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In general terms the closest points for P/V clearance are 10*ATDC as the intake is opening, and 10*BTDC as the exhaust is closing. I use those points to determine if the P/V clearance is even remotely close. If I do a check at those two points and the numbers are near .06 on the intake and .100 on the exhaust then I'll check further.
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