Minimum safe head to piston clearance.

fireblade · Post by **fireblade** » Mon Nov 17, 2008 1:03 am

Having finally hauled my scoot out of the shed for summer, I though I'd make some long awaited mods. I rang the local Honda agents, and to my suprise things like head gaskests aren't too costly.
I've also been planning a pulley mod for ages, (I have a lathe) and I was rereading the tech doc for the 1000th time. I see it refers to the aluminuim pulley, as being from the 84/85 model. I have one of these engines for spares. SO, If I use the head off that engine, AND skim some off it too, what the minimum safe squish clearance?

keithw · Post by **keithw** » Mon Nov 17, 2008 9:07 am

The maximum reccomended head cut is .020". I've done a couple on my lathe. While you have the head off check the piston. If it is a dished piston order a domed one and replace it. Make sure your exhaust port is cleaned of carbon. For the pulley mod you will need to make some spacers to keep the outside half engaging the shaft splines correctly. Putting the spacers behind the inner half will make it go faster and putting them between the halves will return it to stock. I used several thin spacers so I could adjust it. When you put the pulley back together make sure the top of the belt does not extend above the edge of the pulley.

keithw

Kenny_McCormic · Post by **Kenny_McCormic** » Mon Nov 17, 2008 7:44 pm

You can take .020" off, if i had to guess this bring it down to .010" of squish or so, but im probably wrong.

fireblade · Post by **fireblade** » Tue Nov 18, 2008 1:10 am

I'm concerned that if I skim .020 off the 84-85 head, then stick it on my 86 motor the piston to head clearance might be too tight.
Has anyone taken .020 off their head and then measured the clearance? Aren't the head gaskets .020? I could fit my head with no gasket, measure the clearance, then extrapolate how much will be safe to remove from the early head, without getting a tigher clearance than I'd get with the later head with .020 shaved off.

Bear45-70 · Post by **Bear45-70** » Tue Nov 18, 2008 3:20 am

fireblade wrote:I'm concerned that if I skim .020 off the 84-85 head, then stick it on my 86 motor the piston to head clearance might be too tight.
Has anyone taken .020 off their head and then measured the clearance? Aren't the head gaskets .020? I could fit my head with no gasket, measure the clearance, then extrapolate how much will be safe to remove from the early head, without getting a tigher clearance than I'd get with the later head with .020 shaved off.

We always used clay to do that. But a buddy just did his outboard race motor using Silly Putty.

keithw · Post by **keithw** » Tue Nov 18, 2008 9:09 am

The head gasket is about .020" thick. Assemble it without the gasket and see how it does. If it works then shave the .020" off.

keithw

Post by **noiseguy** » Tue Nov 18, 2008 10:20 am

I used lead electrical solder in a cross pattern on the piston to measure assembled and torque'd head to piston clearance. I could never get good measurements with modelling clay... not sure what the technique is supposed to be and it just made a mess.

I'm currently running about .025" head to piston clearance and have had no issues. Stock is about .045" before any head machining.

This is with a '84 Spree head and dished piston.

Kenny_McCormic · Post by **Kenny_McCormic** » Tue Nov 18, 2008 3:51 pm

I wonder how close they can be run without running AVgas.

Bear45-70 · Post by **Bear45-70** » Tue Nov 18, 2008 8:30 pm

Kenny_McCormic wrote:I wonder how close they can be run without running AVgas.

You are gonna need to up the compression to 11 to 1 or better to require more than pump premium gas and I'm not sure you can do that with these two strokes with their porting.

Kenny_McCormic · Post by **Kenny_McCormic** » Tue Nov 18, 2008 9:46 pm

Whats the area of the stock head w/ spark plug installed?

Bear45-70 · Post by **Bear45-70** » Tue Nov 18, 2008 10:48 pm

Kenny_McCormic wrote:Whats the area of the stock head w/ spark plug installed?

Take a stock head with a spark plug installed and invert it. Then use a syringe to fill it with alcohol (isopropyl) and however much you use will give you the answer. If the piston is dished or domed, will change your number of course. Also remember with 2 stroke the ports screw with your compression numbers because the are open for part of the stroke.

fireblade · Post by **fireblade** » Wed Nov 19, 2008 2:18 am

I think the head and piston would crash long before the CR would get high enough to need high octane fuel. Although I have run mine of avgas if that's all I've had at the time. Hasn't done any harm yet. And it smells mighty good too.

Bear45-70 · Post by **Bear45-70** » Wed Nov 19, 2008 3:49 am

fireblade wrote:I think the head and piston would crash long before the CR would get high enough to need high octane fuel. Although I have run mine of avgas if that's all I've had at the time. Hasn't done any harm yet. And it smells mighty good too.

That's because all the AV gas that is available to the public anymore is 100LL. That's 100 average octane and Low Lead. It just isn't that much better than Premium pump gas. Just has detonation resisting additives.

fireblade · Post by **fireblade** » Fri Nov 21, 2008 1:50 am

pehaps I should have been more specific,
In days of yore, (Like about 15 years ago) You used to be able to buy Avgas and the pump at two or three gas stations around my town. BUT, in late 1995/early 1996 when the Govt outlawed leaded petrol for use on the street, (and imposed a fine of $10K for using it on the street) and phased out 96RON leaded pump gas, those gas stations stopped selling it. (Funny that) The gas I was refering to as "Avagas" (Force of habbit from the old days I guess) is actually racegas bought from a fuel supplier near one of the two Speedway tracks where I live. You need to hold a race license to buy it. Weather or not it's some fancy racegas, or just avagas, I don't know. But I do know guys who have purchased their Fuel direct from the aero clubs in our area. You guys might have more than one blend of fuel you can buy, but here we get on size fits all. (Although C16 is available at a premium price) Our nitrous oxide isn't even perfumiated with 9 parts per million sulphur dioxide the way your is. And due to misuse by a small group of individuals, (And a well meaning, but over zealous Govt) you now need to hold a drag license to buy that too.

Bear45-70 · Post by **Bear45-70** » Fri Nov 21, 2008 2:43 am

fireblade wrote:pehaps I should have been more specific,
In days of yore, (Like about 15 years ago) You used to be able to buy Avgas and the pump at two or three gas stations around my town. BUT, in late 1995/early 1996 when the Govt outlawed leaded petrol for use on the street, (and imposed a fine of $10K for using it on the street) and phased out 96RON leaded pump gas, those gas stations stopped selling it. (Funny that) The gas I was refering to as "Avagas" (Force of habbit from the old days I guess) is actually racegas bought from a fuel supplier near one of the two Speedway tracks where I live. You need to hold a race license to buy it. Weather or not it's some fancy racegas, or just avagas, I don't know. But I do know guys who have purchased their Fuel direct from the aero clubs in our area. You guys might have more than one blend of fuel you can buy, but here we get on size fits all. (Although C16 is available at a premium price) Our nitrous oxide isn't even perfumiated with 9 parts per million sulphur dioxide the way your is. And due to misuse by a small group of individuals, (And a well meaning, but over zealous Govt) you now need to hold a drag license to buy that too.

They did that here in the US in the early 70s. Oh FYI, no government is well meaning, only overzealous and power hungry. What is truly best of the citizens has nothing to do with what government does or will do.