Valve Clearance on Larger Camshaft and Flat Top Pistons

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Fish Bite

Fish Bite

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I'll start by saying I understand that valve clearance has many variables tied up in the camshaft profile, camshaft advance, gasket, pistons, etc. and that the only way to know for sure is to measure when mocking up the engine. I will check clearance with a dial indicator when the time comes, but I'm in the planning stage and am hoping to get an idea from anyone who has experience running larger camshafts with flat top pistons.

My concern is that if I buy flat tops and end up needing to cut the reliefs deeper, I will need domed pistons to make up for the lost cc's from the deeper reliefs to stay at the desired dynamic compression. With the way timing goes at my machinist's, I'd likely be outside of the return window for the flat top pistons by the time I buy them, give them to him for his bore/hone work, and he gives me the bored/honed block for mock-up before zero-decking it and balancing the rotating assembly. Then I'm in a situation where I would have to eat the flat tops, buy domed, and now the engine is honed to the flat tops whereas the domed might need a different hone setup.

Here are my current build plans:
-360 0.030" over
-Zero-decked
-Trick Flow heads with 2.02"/1.57" valves and 1.5:1 Harland Sharp rocker arms
-Head gasket of 0.050" is ideal for the desired dynamic compression with the the Trick Flow chambers and cam profile
-The mechanical roller cam I am looking to use is 0.640" valve lift, 283/289 advertised duration, LSA 106, intake centerline 102
-The pistons I'm looking at are forged RaceTec 1000549 flat tops with two valve reliefs. I spoke to RaceTec providing them with the build specs and asking for their input on clearance hoping they had modeling software, and the only thing they told me was that the reliefs are 0.250" deep.

Does anyone have experience with valve clearance using flat top pistons with a similar combination? What's the biggest mechanical roller profile you've been able to pull off with flat tops?
 
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0.640" lift at the lobe?
I don't think that would be possible with 283° advertised duration - I know you can get some pretty crazy profiles with a roller cam, but I think it's more likeley 0.425" at the lobe
 
Makes a lot more sense, Piston to valve clearance has more to do with valve timing than Max lift. Generally it will be closest around 15° either side of TDC. Some cam cards will give you lift at TDC wich can help give you a rough idea without measuring.
Here is a pic from my 318 - pistons 0.014" proud of the deck, felpro 8553 head gasket , SFT cam 230°@0.050", .575" Max lift. Final assembly was done with a thicker 0.051" head hasket so I actually have a little more clearance than shown here:

20230803_174046.jpg
 
Because you asked...

First off, how much compression do you want?

Why would pick a .050 gasket.

Dynamic compression ratio is bullshit.

Max valve lift has nothing to do with P/V clearance.

.250 is PLENTY deep. In fact it's too deep.
 
To your questions, assuming you have had to cut the reliefs deeper to get clearance. Would you need to have a piston with a pop up to regain compression? No, go thinner head gasket. Cometic makes an .029 gasket with other thicker options before you get to .050. That being said a pop up can help with quench.
 
Ted265 said:
Makes a lot more sense, Piston to valve clearance has more to do with valve timing than Max lift. Generally it will be closest around 15° either side of TDC. Some cam cards will give you lift at TDC wich can help give you a rough idea without measuring.
Here is a pic from my 318 - pistons 0.014" proud of the deck, felpro 8553 head gasket , SFT cam 230°@0.050", .575" Max lift. Final assembly was done with a thicker 0.051" head hasket so I actually have a little more clearance than shown here:
Click to expand...
Thanks. That's a helpful comparison. Do you have either the cam model so I can look it up or the cam card so I can get an idea of when the intake valve closed?
 
furrystump said:
To your questions, assuming you have had to cut the reliefs deeper to get clearance. Would you need to have a piston with a pop up to regain compression? No, go thinner head gasket. Cometic makes an .029 gasket with other thicker options before you get to .050. That being said a pop up can help with quench.
Click to expand...
Thanks. My thinking with not relying on the head gasket to increase static compression if I lose some with cutting deeper reliefs was that I'd be battling myself to a degree, in that I'm cutting reliefs deeper and losing some of that cut by bringing the valves back down closer to those reliefs with a thinner gasket. There's going to be more cc change in the head gasket change than the relief change, however, so I could find the sweet spot where the additional relief to account for the valves and thinner gasket will net me the static compression I had before the relief and gasket change.
 
Fish Bite said:
I'll start by saying I understand that valve clearance has many variables tied up in the camshaft profile, camshaft advance, gasket, pistons, etc. and that the only way to know for sure is to measure when mocking up the engine. I will check clearance with a dial indicator when the time comes, but I'm in the planning stage and am hoping to get an idea from anyone who has experience running larger camshafts with flat top pistons.

My concern is that if I buy flat tops and end up needing to cut the reliefs deeper, I will need domed pistons to make up for the lost cc's from the deeper reliefs to stay at the desired dynamic compression. With the way timing goes at my machinist's, I'd likely be outside of the return window for the flat top pistons by the time I buy them, give them to him for his bore/hone work, and he gives me the bored/honed block for mock-up before zero-decking it and balancing the rotating assembly. Then I'm in a situation where I would have to eat the flat tops, buy domed, and now the engine is honed to the flat tops whereas the domed might need a different hone setup.

Here are my current build plans:
-360 0.030" over
-Zero-decked
-Trick Flow heads with 2.02"/1.57" valves and 1.5:1 Harland Sharp rocker arms
-Head gasket of 0.050" is ideal for the desired dynamic compression with the the Trick Flow chambers and cam profile
-The mechanical roller cam I am looking to use is 0.640" valve lift, 283/289 advertised duration, LSA 106, intake centerline 102
-The pistons I'm looking at are forged RaceTec 1000549 flat tops with two valve reliefs. I spoke to RaceTec providing them with the build specs and asking for their input on clearance hoping they had modeling software, and the only thing they told me was that the reliefs are 0.250" deep.

Does anyone have experience with valve clearance using flat top pistons with a similar combination? What's the biggest mechanical roller profile you've been able to pull off with flat tops?
Click to expand...
Put it together. You’ll never hit with .250 deep valve pockets on a wedge head with flat tops at zero deck. What’s the duration at .050?
 
You’re overthinking it also, which is much better than under thinking it, but I think you’re still getting caught in the minutia. The few CCs you would give up cutting the valve pockets deeper wouldn’t affect the compression ratio enough to make a machining change. Like the others have said a slight change in gasket thickness will easily account for it.
 
Fish Bite said:
Thanks. That's a helpful comparison. Do you have either the cam model so I can look it up or the cam card so I can get an idea of when the intake valve closed?
Click to expand...


You are going down a bad road.

With the head gasket and the valves lashed, the intake valve needs whatever your piston to HEAD clearance is plus .010-.015. Any more than that and you are hurting power. That's a FACT.

For example, let’s say you have .050 piston to head clearance with the gasket. That means all the clearance you need at the most is .065.

If you were using a quality gear drive you could get that all the way down to whatever your piston to head clearance is. But I doubt you are using a GD.

For the exhaust you need, including lash and the gasket is 1.6-2.0 time your intake clearance.

So…that means if you are the nervous type and you have .065 intake valve clearance you’ll need .104-130 and I run less than the 1.6 and never bent a valve.

In fact, my engine right now has the exhaust valve just touching the carbon on the piston. This is a street/strip car so I’m going to take a .010 cut off the face of each valve.

As TT says above, with those valve pockets you’ll have way too much clearance.
 
251/257 on a 106 is big enough to make sure it doesn’t touch but again with your .250 valve pockets it ain’t gonna.
 
Newbomb Turk said:
You are going down a bad road.

With the head gasket and the valves lashed, the intake valve needs whatever your piston to HEAD clearance is plus .010-.015. Any more than that and you are hurting power. That's a FACT.

For example, let’s say you have .050 piston to head clearance with the gasket. That means all the clearance you need at the most is .065.

If you were using a quality gear drive you could get that all the way down to whatever your piston to head clearance is. But I doubt you are using a GD.

For the exhaust you need, including lash and the gasket is 1.6-2.0 time your intake clearance.

So…that means if you are the nervous type and you have .065 intake valve clearance you’ll need .104-130 and I run less than the 1.6 and never bent a valve.

In fact, my engine right now has the exhaust valve just touching the carbon on the piston. This is a street/strip car so I’m going to take a .010 cut off the face of each valve.

As TT says above, with those valve pockets you’ll have way too much clearance.
Click to expand...
Back in the day , Chrysler and most everybody else said , minimum of .080 on intakes , and minimum or .100 on exhaust . This was for performance engines (dragstrip) , but "DONT FLOAT THE VALVES very hard , if at all !!!
I think you`ll have more than that to start with...jmo
 
famous bob said:
Back in the day , Chrysler and most everybody else said , minimum of .080 on intakes , and minimum or .100 on exhaust . This was for performance engines (dragstrip) , but "DONT FLOAT THE VALVES very hard , if at all !!!
I think you`ll have more than that to start with...jmo
Click to expand...


I’m sure the OP will have way more than that so he’s good to go.

I can’t of a reason why he’d ever need to notch those pistons.
 
Fish Bite said:
Thanks. That's a helpful comparison. Do you have either the cam model so I can look it up or the cam card so I can get an idea of when the intake valve closed?
Click to expand...
Here is a photo of the cam card, kinda wish the releifs were shallower on the KB167s.
20220119_185333.jpg
 
Thanks everyone for your input. I'll plan on moving forward with the cam profile and thinner head gasket without worry about clearance issues.

For what it's worth, RaceTec reached back out today when I didn't expect it after our previous conversation. They crunched numbers and confirmed the valves will clear, as many of you already knew.

Looking at different off-the-shelf options, the only forged flat-top piston I can find with reliefs smaller than 5 cc is DSS 16350-4030 with 3 cc reliefs. However, much of the info I find online about DSS Mopar forged pistons is that they are heavier and shoddier manufacturing than other brands. People compare them to TRWs with a worse finish. Aside from going with custom pistons, unless someone knows of another option, it looks like I'll be stuck with the 5 cc 0.250" deep reliefs.
 
Fish Bite said:
Thanks everyone for your input. I'll plan on moving forward with the cam profile and thinner head gasket without worry about clearance issues.

For what it's worth, RaceTec reached back out today when I didn't expect it after our previous conversation. They crunched numbers and confirmed the valves will clear, as many of you already knew.

Looking at different off-the-shelf options, the only forged flat-top piston I can find with reliefs smaller than 5 cc is DSS 16350-4030 with 3 cc reliefs. However, much of the info I find online about DSS Mopar forged pistons is that they are heavier and shoddier manufacturing than other brands. People compare them to TRWs with a worse finish. Aside from going with custom pistons, unless someone knows of another option, it looks like I'll be stuck with the 5 cc 0.250" deep reliefs.
Click to expand...

I've used DSS pistons and shoddy they are not.
 
Once you move ahead, I would definitely check valve to piston with checker springs/dial indicator to see where you are at (forget clay).

famous bob said:
Back in the day , Chrysler and most everybody else said , minimum of .080 on intakes , and minimum or .100 on exhaust . This was for performance engines (dragstrip) , but "DONT FLOAT THE VALVES very hard , if at all !!!
Click to expand...

I always went by 0.080 minimum on intakes, 0.120 minimum on exhaust. I was always insane (7200 rpm plus) and I never hit an intake at real close to 0.080, but I did tap the piston carbon on one exhaust valve at 0.109 clearance. Just a data point to consider.
 
When using solid roller lifters, here is a veeeery easy way to check V to P clearance. If you anticipate that you have plenty of V to P clearance, then assemble the engine with the springs you intend to use. Easier with two people. Adjust valve lash & rotate engine until the cyl to be tested is at TDC, on overlap. Simply get a pair vice grips or multigrips & lever up on the prod side of the rocker to push the valve down until it touches the piston. The valve travel is your V to P clearance. When clearance gets tight, you need to check a few degrees before TDC on the exh valve & a few degrees after TDC on the intake valve. You can set up a dial gauge to take the measurement, but I just use a vernier caliper.
 
DC 340 said:
Once you move ahead, I would definitely check valve to piston with checker springs/dial indicator to see where you are at (forget clay).
Click to expand...
Absolutely. I use checking springs and a dial indicator and rotate the crank until I find the tightest clearance. I perform the test with the degree wheel still on the crank after finding TDC and degreeing the cam, and I write the clearance and at what degree on the cam card in the event I need to reference them again.
 
I’ve used DSS pistons also and they measured as good or better than most pistons Ive used. The Mahle motorsports forged pistons were the nicest I’ve ever measured
 
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famous bob said:
Back in the day , Chrysler and most everybody else said , minimum of .080 on intakes , and minimum or .100 on exhaust . This was for performance engines (dragstrip) , but "DONT FLOAT THE VALVES very hard , if at all !!!
I think you`ll have more than that to start with...jmo
Click to expand...
you need to clay check it , that way you`ll get the closest reading at the tightest of stage of rotation , fail proof !!
 
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