Head clearance. Nova DD 21

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Samuel Hagan JR

Well-Known Member
Joined
Jul 18, 2010
Messages
4,567
Been tuning out of the box head clearance for a while and it was .028”. The motor broke in nice and great. Lowered it to .017” Gained quite a bit of speed. I don’t want to over do but what are some other people running for head clearance on there 21 DD Nova
 
Been tuning out of the box head clearance for a while and it was .028”. The motor broke in nice and great. Lowered it to .017” Gained quite a bit of speed. I don’t want to over do but what are some other people running for head clearance on there 21 DD Nova
I'm running .007
Walt Barney
 
Been tuning out of the box head clearance for a while and it was .028”. The motor broke in nice and great. Lowered it to .017” Gained quite a bit of speed. I don’t want to over do but what are some other people running for head clearance on there 21 DD Nova
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Dam are you subtracting
Been tuning out of the box head clearance for a while and it was .028”. The motor broke in nice and great. Lowered it to .017” Gained quite a bit of speed. I don’t want to over do but what are some other people running for head clearance on there 21 DD Nova
Sam are you subtracting the .015” For the dish in the top of the piston? Is this the same DD you broke a plug in? Why would anyone run .028” clearance on any nitro motor?
Even .017” is high.
.008” is a safe strong number. You can go tighter but will need to monitor it more.
 
No I’m not subtracting the dish. Just measuring top of Piston to bottom of head button. I did all this late night so I may be off.
 
No I’m not subtracting the dish. Just measuring top of Piston to bottom of head button. I did all this late night so I may be off.
Sam,
Danny is right. If you are measuring center of piston (most accurate method) you have to allow for the dish top piston. I have had several boaters adjust for what they thought was a .006-.008 clearance, just to find out later they were negative and ended up knocking the bottom out of the rod. When you are lowering the head to these numbers, a good head gauge measuring device is imperative. There are several standard and turbo adapters available by manufacturers such as Speedmaster. Let me know if I can help.
 
I’m okay with measuring. I just wanted to see what everyone was measuring there head clearance at. I guess some people thought I was measuring the dishes part. Which I’m not. As stated before the out of the box head clearance was .028”. I guess that was for break in.
 
Hi Sam,

You may or may not realize this but the piston on that engine (and a few other higher end novas) have a different shaped top. Not only does it have a dish in the middle, but at the outside of that dish (where a typical dished piston would be be flat) it's actually angled down. So if you took that piston out of the engine and put it upside own on a table, the middle would touch the table but the outside would not. I'm not a 100% sure on the DD but I've also found that the squish angle on the head and the angle on top of the piston are not always the same either. This can make it tricky to measure the head clearance because it can be tight in one spot and not in another across that area.

That being said, I've found it to be best to measure the head clearance on an engine like this using plasti-gauge. There's obviously other ways to skin the cat here, but to me this covers all the bases in one shot. It will tell you the worse case clearance across the bore if the squish angle is different than the top of the piston also. Using a dial indicator like Ron mentioned is probably fine too, I just haven't compared the two methods with a novarossi dished piston yet.

The clearance you have now is extremely high. I typically break in a 21 at 0.012" and then drop it down to 0.008". As Danny said, you can go lower, but you need to keep an eye on your plug and needle. You'll be running richer anyway but if you accidentally go too lean and burn a plug - at 0.006" you'll most likely find pieces of the element imbedded in the squish band of the head afterwards. Don't get me wrong, less clearance is better but in my opinion it depends on what you are trying to do. I find that 0.008 works just fine for me in district racing.

Sorry for the novel, but I hope this helps.

Brian
 
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Walt I appreciate the help. I’ll give you a call this weekend if your not busy. I also have a few tank I need to send you to check out.
 
I appreciate all the help. It’s been a learning curve to me. It seems this motor is a lot more temper mental then my Valvola but that’s my opinion.
Other then that once I found a good needle setting and drop the head clearance down we found a lot more speed. I may keep the clearance where it’s at. It runs great and Ron is out of stock on Rods so I’d rather not push it. I ordered a extra head button while he had 1 in stock.
 
It's your call Sam but I'd at least drop the head down to 0.012". That's still very high (and safe). You will also be able to run much richer and probably end up with more power / speed and cooler engine temps than you have now.

What percent nitro are you running?

Brian
 
It's your call Sam but I'd at least drop the head down to 0.012". That's still very high (and safe). You will also be able to run much richer and probably end up with more power / speed and cooler engine temps than you have now.

What percent nitro are you running?

Brian

That sounds more reasonable. I’m running 50 percent.
You brought up engine temps. Will higher head clearance cause higher temps. I have had issues with engine temps before and I’m pretty sure But not completely sure it was because it’s of not enough fuel.
 
That’s my point Sam. They don’t come with .028” clearance.
if there is .028” of shims the clearance is roughly .013”
 
i have found the only way to accurately measure a dd is with a dial or digital gauge. put the gauge in the button,turn the sleeve upside down and install piston,put the head button in the sleeve and shove the piston against the head, then zero the gauge. all the liner does is make sure the piston and head are in alignment. next assemble the engine with gauge and read the clearance at tdc. i use an old water jacket with a slot in it so i don't have to remove the gauge from the button to assemble
 
If I take all the shims out the Piston will be hitting the head button. How is that .013” of clearance.
 
I’m probably doing it wrong. I’ll recheck everything once more. I’m done racing for the season so it’s not like I’m in a rush. Thanks guys.
 
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