discussion of head button squish angle

dwilfong · Nov 27, 2017

delete

Michael Caruso · Nov 28, 2017

Jim Allen said:
When we build high performance nitro racing engines there are different diameter head buttons machined in .0002" increments. The reason is because we do not attempt to hone cylinders to any exact size, only to an exact taper & very round. A loose fitting head button (.0005" +) will cause a nitro engine to go into detonation when running at WOT. This would also apply to a gas engine that has the head protruding down into the cylinder.

Jim Allen

Hi Jim,

I have spent many hours looking at all you have built and shared with us. AWESOME work my friend and thank you for posting. I have a soft spot for your geared twin as I built one many moons ago with then new K&B 6.5's family issues kept me from firing it up. Maybe again someday. I have something that I have used on the O.D. of the head which drops into the cylinder that reduced Detonation. Now, I did it many many years ago no Dyno but you could prove or disprove sometime when you are testing and time permits. What I did was first put a few grooves (nose of tool bit) into the head section which dropped into the cylinder and it helped reduce Detonation. Next I had a square tool bit ground up and re-machined the grooves this worked much better.

I got the idea from an old outboard guy who used to add square grooves into the I.D. of a reed cage used on 1950 7.5 hp Mercury's. In that application it helped reduce the pressure in-balance between cylinders. Early built engines just used a bored hole that the crankshaft passed through.

Just like the anti-Detonation grooves used for years between the top and second ring for racing engines. The OEM people knew it for years before racers and used larger second ring end gaps or increased lateral second ring to ring land clearance. Both worked just fine. Cummins Diesels use .026 top .059 sec ring gaps.

Got a little off topic.

Jim, Do you think the tighter fit just transfers heat from the chamber to the sleeve better reducing Detonation?

Mike

Jim Allen · Nov 28, 2017

Hi Mike,

Any valid information is always valuable when attempting to design & build a proper functioning high performance two cycle engine. Since detonation will prevent any engine from going higher in the amount of HP that can be developed, preventing that detonation becomes critical. All of my mentors, who were engine builders, believed that you must be able to cut various metals, knowing the exact number that is cut. Without knowing positively what the number is, makes it very difficult to know what is working & what is not working. This becomes especially true when the engine size becomes smaller as is the case of our miniature two cycle engines.

As for the geared twin project, 90% of the essential work has been completed; 3 sets of gears (+ or - .0002" gear to gear & .+ or - 0002" on the OD) with two masters made of #4340 unleaded, hardened steel; 2 engines with hardened steel front ends & steel roller rods; .625" bore automatic fuel metering carburetors; machined tuned pipes; ball & pin universals; special machined strut assembly; hardened steel rudder & skid fin; 2 machined engine mounts that are one piece holding both engines at the same time etc. The engines use 3 spur gears in a 1 to 1 ratio & one 9/32" drive shaft fastened to the middle gear. Gearing the engines 1 to 1, doubles the torque, doubles the HP & keeps the engine RPM's the same (minus gear losses) at the single 9/32" drive shaft. The engines develop 7.5 HP at 30,000+ RPM. Everything has been thoroughly tested & I have a special hull built by Andy Brown.

I don't think the tighter fit helps much with heat being transferred to the cylinder. It certainly would have an effect if it was pressed into the cylinder, but this is not possible. Consider how much more heat is being transferred with the large lip liner, we have discussed previously, which is bolted directly to the head button & the entire surface of the upper engine crankcase.

Jim Allen

Michael Caruso · Nov 29, 2017

Jim Allen said:
Hi Mike,

Any valid information is always valuable when attempting to design & build a proper functioning high performance two cycle engine. Since detonation will prevent any engine from going higher in the amount of HP that can be developed, preventing that detonation becomes critical. All of my mentors, who were engine builders, believed that you must be able to cut various metals, knowing the exact number that is cut. Without knowing positively what the number is, makes it very difficult to know what is working & what is not working. This becomes especially true when the engine size becomes smaller as is the case of our miniature two cycle engines.

As for the geared twin project, 90% of the essential work has been completed; 3 sets of gears (+ or - .0002" gear to gear & .+ or - 0002" on the OD) with two masters made of #4340 unleaded, hardened steel; 2 engines with hardened steel front ends & steel roller rods; .625" bore automatic fuel metering carburetors; machined tuned pipes; ball & pin universals; special machined strut assembly; hardened steel rudder & skid fin; 2 machined engine mounts that are one piece holding both engines at the same time etc. The engines use 3 spur gears in a 1 to 1 ratio & one 9/32" drive shaft fastened to the middle gear. Gearing the engines 1 to 1, doubles the torque, doubles the HP & keeps the engine RPM's the same (minus gear losses) at the single 9/32" drive shaft. The engines develop 7.5 HP at 30,000+ RPM. Everything has been thoroughly tested & I have a special hull built by Andy Brown.

I don't think the tighter fit helps much with heat being transferred to the cylinder. It certainly would have an effect if it was pressed into the cylinder, but this is not possible. Consider how much more heat is being transferred with the large lip liner, we have discussed previously, which is bolted directly to the head button & the entire surface of the upper engine crankcase.

Jim Allen

Thanks Jim

Michael Costanzo · Nov 29, 2017

bermudadude said:
"thanks jim for the explanation. i have to ask while we are talking about heads.. i just noticed that on this OS Max VRM .81, the head button from the factory swims around the bore of the sleeve?? there is space between the side of the button and the sleeve?? when the bolts go thru the head and into the case,everything lines up.. i have never seen anything like this..any motor i have worked on the head button fits tight in the bore. what is the story here?? thanks,mike."

simple- u got a bad one. i have os81/91's they all tight. also ive never seen a loose one on any engine of the dozens n dozens of all brands of marine racing nitro that i have

the strange thing is that all the holes drilled in the button fit perfectly and line up 100% to the case??? so your telling me the .67 and the .81 button's are the same?? if that's the case,how do i get my hands on a .81 button? thanks,mike.

Michael Costanzo · Nov 29, 2017

Jim Allen said:
When we build high performance nitro racing engines there are different diameter head buttons machined in .0002" increments. The reason is because we do not attempt to hone cylinders to any exact size, only to an exact taper & very round. A loose fitting head button (.0005" +) will cause a nitro engine to go into detonation when running at WOT. This would also apply to a gas engine that has the head protruding down into the cylinder.

Jim Allen

thank you for the reply. i thought i was loosing my mind....