Head Clearance discussion

[Greg Bonahoom]

What are the benefits/consequences of raising or lowering head clearance? Lets assume I have a novarossi.21 motor and running 65% nitro. The consensus on this forum seems to be that I should run around.008” clearance. What benefits/consequences would I see if I ran at .010 or .006”? I seem to notice that I blow plugs easier at a lower clearance. I am just curious about the theories on this and what others have found through testing.

 

[LohringMiller]

There are two areas in a head, the combustion chamber area and the squish area. The squish area is the band around the outside of the central volume. You want to keep the squish clearance as close as possible to limit detonation. You need to adjust the combustion chamber volume to get the right compression for ignition. Head shims adjust both and are OK for fine adjustments, but not the ideal method to set either the volume or the squish clearance.

Lohring Miller

 

[David Ashcroft]

If you run .006" on the 21 size you will benefit with torque,as they rev well anyway.but don't forget to check rod pin / bushing wear because over time if running tight.006" the piston may infact be slapping the button. .

 

[dwilfong]

with a stock head close it down till it takes plugs then back off 1....... With out changing the chamber design or squash angle you have no other recourse.
Now if you can make your own head buttons there is a whole different world you can play with to optimize the head.
I have a TON of heads that I have tried. Chamber shape has a lot to do with how the flame travels in the chamber.
I was told by a good boater that burn rate is king. But with a good chamber design you do not need as much burn rate to complete burn the fuel load. so you do not have to lean on it so much.
You can change burn timing with plug heat range but not the rate at witch it burns. Now you can light off more at one time with a hotter plug but it will still burn at the same rate.
So what chambers work best??????
Lets hear some response to this...........Should be a good point to talk about.
Bin a long time since we had a good topic to ponder on here.
My new shop is almost done and will be making some heads for a buddy here soon.
He can't get his new RS45EVO to spool up hard with out taking the plug.

 

[Grimracer]

Greg, Looks like this is starting to get off track.

Most here have NO idea what the benefit of .002" change.. ether direction.... truth is its like the flow meter discussion.. my advice is.. set the head space with your intended fuel, prop and plug. I would set mine so the boat would launch well.. for most of my setups that was about .012". CMB LS 21 green head, OPS 3280 and H10 prop. BTY.. I set my head space AFTER I had the pipe length set..

Just my way.. the good old pond dyno!

Grim

 

[Terry Keeley]

I set everything about 0.012", much lower and I see a nice little imprint of the squish band on the piston. I had to back my VAC 90 off another couple thou because of this. I run 60% in everything, cut the water down a bunch and run hot plugs.

These heads run great on my little geared mono: https://www.intlwaters.com/threads/on-line-head-volume-calculator.74796/

 

[Jim Allen]

Attempting to determine what is the best deck clearance, chamber volume, chamber shape, squish angle, etc., etc., becomes difficult or maybe impossible without understanding how the tuned pipes or exhaust systems attached to these engines work. For piped engines, the wrong tuned pipe volume, wrong chamber shape, wrong stinger length & inside diameter will be some of the things that affect what fuel & how a fuel can be used!!

Jim Allen

 

[John Beardslee]

I set everything about 0.12", much lower and I see a nice little imprint of the squish band on the piston. I had to back my VAC 90 off another couple thou because of this. I run 60% in everything, cut the water down a bunch and run hot plugs.

These heads run great on my little geared mono: https://www.intlwaters.com/threads/on-line-head-volume-calculator.74796/

Terry. I think you mean .012”, .12 would be almost an 1/8” or a touch over 3mm, that’s a lot of head space. Typo right. Or we’re you just checking to see if anyone really reads this stuff. You are right on with the vac91 engine, the extra .002 added to my original .012 probably would have saved me burning a hole in the piston on my twin, the other engine didn’t look good either when I tore it down.

 

[Terry Keeley]

Lol, yup fixed.

How was that inside motor after Flint? Didn't feel too healthy to me. Scuffed piston?

 

[PaulHail]

It is helpful to have an understanding of the purpose of the squish band as well as understand that the squish clearance cannot be addressed by itself, it needs to be considered in relation to the head volume, fuel and squish area.

The purpose of the squish band is to create a high velocity swirl in the combustion chamber which both aids in combustion and prevents detonation, or pre-ignition. The squish velocity is affected by the area of the squish band, the squish clearance and engine rpm. In a practical sense there is a limit to how tight you can set the squish as the piston will start hitting the squish band when it's set too tight.

The thing to realize is if you just lower the squish which will increase the squish velocity, which is good to a point to protect against detonation, you are also increasing the compression ratio which will increase detonation. so if you want to run as tight a squish as possible, you may also need to machine out the combustion chamber to keep the compression ratio at a volume that will not cause detonation.

 

[John Beardslee]

Lol, yup fixed.

How was that inside motor after Flint? Didn't feel too healthy to me. Scuffed piston?

I sent you pictures in a text, holed the piston! Really ugly, first time. Liner is perfect with no damage.

 

[Jim Allen]

Holed or pockets in the middle of a piston crown can come from several different things. Here is a picture of a piston crown with a pocket melted in the middle. During this engine test in 1987, the center post of the glow plug began to glow red. Shutting the engine down prevented the piston from being holed. Notice the two red lines on the piston's crown which show the angles of the vapor trails from the incoming main transfer charges. The radial & axial angles of the main transfers plus the thickness of the cylinder wall determine the direction of these gas flows. Their direction can & should be used to cool the piston crown in its hottest area, the center. I make no mention of what happens when the tuned pipes volume, in relation to the displacement of the engine, is to small. Changing the radial angles of the main transfer walls closest to the exhaust window eliminated this problem in this engine.

Jim Allen

 

[Terry Keeley]

View attachment 275338
I sent you pictures in a text, holed the piston! Really ugly, first time. Liner is perfect with no damage.

Wow, no wonder it wasn't running well! Leeeeeaaaannn!!!

Here's a pic I found somewhere...

 

[Alan Elzer]

How do you measure head clearance & volume when having a dome piston & dish top in piston. An does this really help the engine..if so how ? I have this on new keep 21 outb

 

[John Beardslee]

How do you measure head clearance & volume when having a dome piston & dish top in piston. An does this really help the engine..if so how ? I have this on new keep 21 outb

Alan, I use a dial indicator with a fitting for what ever plug thread you are using and with the head and piston out of the engine I hold the piston sold against the head and set the indicator to zero. Then after re-assembling the engine take another reading to see what it reads at TDC and adjust the head clearance accordingly

 

[dwilfong]

Try calculating with a .45cc chamber and .009 clearance.

 

[timothy johnson]

You don't mention what plugs you were using, my experience with both the RS and V-5 is they like OD 289s turbo and standard. I run 60% with a .009 to .010 HC on both motors and rarely take out a plug. I also always pull the element out on the plugs which with my 67 motors is the difference between burning plugs or not. When using other brands of plugs, which I won't mention, plug life is not always good. In the last couple of years Ive learned that a balanced crank, a slight increase of the closing side of the drum or disk timing along with a thorough cleaning of the case and ports really makes a motor. My real real speed has come with Bouchie pipes, CMB nitro pipes and props from Mark and Brian Buass. I guess for me with CMB "stock really does work"

 

[Timbo Whalen]

You don't mention what plugs you were using, my experience with both the RS and V-5 is they like OD 289s turbo and standard. I run 60% with a .009 to .010 HC on both motors and rarely take out a plug. I also always pull the element out on the plugs which with my 67 motors is the difference between burning plugs or not. When using other brands of plugs, which I won't mention, plug life is not always good. In the last couple of years Ive learned that a balanced crank, a slight increase of the closing side of the drum or disk timing along with a thorough cleaning of the case and ports really makes a motor. My real real speed has come with Bouchie pipes, CMB nitro pipes and props from Mark and Brian Buass. I guess for me with CMB "stock really does work"

We've been running .008"-.010" clearance on our 45 engines, depending on weather. Like you, we like the OD 289's, but the last couple

of orders we've been getting 1-2 dead plugs from each order of 10- right out of the box. We've found the Rossi Std. and Turbo plugs to be

on par with OD, plus the Rossi's have much better selection for temp. range when going cold.

We haven't found a better pipe for the 45 engines than the NR #50901 (we drill stinger w/Z bit- .413") and we ceramic coat pipe and header,

and we run the pipe short and fat on 60%. Try one on your CMB's at 9.25"-9.5" and see what you think... I think the difference is in the

2 stage converging (baffle) cone.

For the money involved, that Bouchie pipe has nothing on NR nitro pipe with a full mod 46DD engine (we're running 193.8° Exh.).

These engines, either NR or CMB, really don't need any exotic trickery for a winning heat racer- just deburr, align things up, delay intake

closing 2°-3° and try to get as close to 28°-30° blow down as you can, without hogging on the exhaust port more than a couple of degrees.

I like those Steve Wood liner shims to test with on a day when the air is really good, too.

Thanks-

TW

 

[julianconde]

We've been running .008"-.010" clearance on our 45 engines, depending on weather. Like you, we like the OD 289's, but the last couple

of orders we've been getting 1-2 dead plugs from each order of 10- right out of the box. We've found the Rossi Std. and Turbo plugs to be

on par with OD, plus the Rossi's have much better selection for temp. range when going cold.

We haven't found a better pipe for the 45 engines than the NR #50901 (we drill stinger w/Z bit- .413") and we ceramic coat pipe and header,

and we run the pipe short and fat on 60%. Try one on your CMB's at 9.25"-9.5" and see what you think... I think the difference is in the

2 stage converging (baffle) cone.

For the money involved, that Bouchie pipe has nothing on NR nitro pipe with a full mod 46DD engine (we're running 193.8° Exh.).

These engines, either NR or CMB, really don't need any exotic trickery for a winning heat racer- just deburr, align things up, delay intake

closing 2°-3° and try to get as close to 28°-30° blow down as you can, without hogging on the exhaust port more than a couple of degrees.

I like those Steve Wood liner shims to test with on a day when the air is really good, too.

Thanks-

TW

post the picture of the pipe your talking about I’m curious. I run my 45 RS Evos with OD99’s, CMb nitro pipe at 9- 9 1/4 and it hauls ass on my twin of course. but they are full modified engines but no need to change the exhaust timing it’s stock everything else is not stock

 

[Timbo Whalen]

post the picture of the pipe your talking about I’m curious. I run my 45 RS Evos with OD99’s, CMb nitro pipe at 9- 9 1/4 and it hauls ass on my twin of course. but they are full modified engines but no need to change the exhaust timing it’s stock everything else is not stock

Bud Connor at East Coast Powder Coating does my ceramic coating and DFL on pistons. He knows 2 strokes and is well known

on the Outboard Drag Boat scene... he knows Merc V6's as good as anyone.