Piston Deck Height

[swooshdave]

I’ve been having problems with people commenting on my pictures that things don’t look right. I think part of it’s that I’m using dramatic lighting at angles people aren’t use to seeing their bikes at. The other parts is that I’m messing up.

I haven’t measured it yet but I think spec is .050”.

If it’s not do I need to pull the barrel and put a thicker base gasket in? I’ve got the standard one in there now?

 

[comnoz]

I’ve been having problems with people commenting on my pictures that things don’t look right. I think part of it’s that I’m using dramatic lighting at angles people aren’t use to seeing their bikes at. The other parts is that I’m messing up.

I haven’t measured it yet but I think spec is .050”.

If it’s not do I need to pull the barrel and put a thicker base gasket in? I’ve got the standard one in there now?

If your using a standard head your probably good to go.

If your using a Combat head I can send you a 1/16 copper base gasket to get the compression down to reasonable. Jim

 

[swooshdave]

If your using a standard head your probably good to go.

If your using a Combat head I can send you a 1/16 copper base gasket to get the compression down to reasonable. Jim

Initial measurement, just eyeballing TDC, no dial indicator, is .034”.

What is the Combat spec for how far above the deck you can be?

 

[Time Warp]

If you have a vernier why not snug the cylinder down if its not already, measure the piston to cylinder deck inline with the pin and head gasket face to counterbore.
Minus one from the other plus the thickness of the head gasket you are going to use (easier if it is copper) and that will be the clearance of hard parts.
If you have a measurement of 0.040" you should have no problem.

 

[swooshdave]

If you have a vernier why not snug the cylinder down if its not already, measure the piston to cylinder deck inline with the pin and head gasket face to counterbore.
Minus one from the other plus the thickness of the head gasket you are going to use (easier if it is copper) and that will be the clearance of hard parts.
If you have a measurement of 0.040" you should have no problem.

I’ll take a more accurate measurement in a bit.

Head is waiting for the rockers before it can go back on.

 

[comnoz]

I’ll take a more accurate measurement in a bit.

Head is waiting for the rockers before it can go back on.

The height above the deck is the same for Combat or standard. The difference is in the amount cut off the cylinder head.

Your piston protrusion looks about right to my eye but if you use a Combat head I would recommend a spacer under the barrel or you will be risking detonation with today's fuels.

 

[swooshdave]

I took a more accurate measurement. I get .043". I didn't have a problem with pinging previously and I always use premium. Is it more of a problem at higher elevations?


I have a new composite head gasket and a used copper one. Can you get the copper hot enough with propane or MAP gas to anneal?

 

[dynodave]

.050 Above deck is what I got 30 years ago for my combat. And appears to be the same for all 20M3 and up 750.
old school head gasket .035 approx
Combat squish band/counterbore is approx (.120 normal - .040 cut for combat) = .080 Same thing on all unmodified C heads I have ever handled.


#1 Unfortunately, If you run enough octane and good mixture ratio, the ignition curve of the selected generic ignition is probably the source of pinging .
#2 Unfortunately When someone declares the timing is "spot on", the ignition curve discussions are usually well below the threshold for joining in. Ignition is a process across the whole rpm range ...not a spot at 5000 rpm.
Heck even in 1973, a stone age company such a chevrolet had probably a dozen different distributor curves listed in my brand new 73 chevy factory shop manual.

 

[swooshdave]

@dynodave I can’t tell what your recommendation is? Lower the compression, lower the pistons, get a better ignition or just making comments. The emoticons are throwing me off.

 

[comnoz]

For compression you can get by with the high Combat compression ratio as long as you are also running the Combat cam and you are careful with the fuel.

The problem I have seen is gas stations are not always honest with the fuel they sell. [Or maybe some delivery driver put the fuel in the wrong tank]

If your not running the Combat cam you will need to lower the compression.

Mapp gas will anneal a gasket. A burner on a gas stove or grill will also work.

It's pretty tough to get the gasket evenly annealed with just a propane torch but it can be done.

 

[jseng1]

If you want to adjust compression use a thicker copper head gasket so you don't have to remove the barrels.

Annealing your copper head gasket won't make any difference because it won't crush or change dimension. If you want to make sure it doesn't leak oil then use pliobond (nothing else) and apply it to each surface (4 surfaces) because it must bond to the metal (not just bond to one surface and clamp against another). Use a loop of .005" diameter copper wire around the pushrod tunnels and oil return hole.

See dyno daves .050" piston protrusion and squish band measurements above - they are correct.

 

[acotrel]

Are you worried about the comp. ratio or the squish band clearance ? Your fuel sets the maximum comp. ratio you can use. And the squish band clearance has to be sufficient to cope with the stretch in your engines internals at max. revs. Modern fuels should cope with 11 to 1 comp. ratios as long as you don't jet too lean or use too much ignition advance.

 

[rvich]

Would it be good to measure the squish band before deciding whether to use a thicker base gasket versus a thicker head gasket?

 

[jseng1]

Would it be good to measure the squish band before deciding whether to use a thicker base gasket versus a thicker head gasket?

Yes
See dyno daves measurements above. Lowering the head .040" increases C.R. roughly 1 point (from 9:1 to 10:1).

 

[comnoz]

Would it be good to measure the squish band before deciding whether to use a thicker base gasket versus a thicker head gasket?

A thicker base gasket or a thicker head gasket will have the same affect on the squish clearance and compression.

 

[rvich]

I guess that should have been obvious! It's why this forum needs guys like me around. Someone has to ask the stupid questions!

 

[Matt Spencer]

Not to mention the pushrod lengths after such meanderings .

 

[MichaelB]

@dynodave I can’t tell what your recommendation is? Lower the compression, lower the pistons, get a better ignition or just making comments. The emoticons are throwing me off.

I think that translates to Boyer blows.
He’a not a fan..

 

[comnoz]

An analogue Boyer is likely to make the motor less sensitive to fuel octane.

The advance curve from an analogue ignition is going to be lazy through the midrange where the motor is going to be most sensitive.

 

[rvich]

I guess what I should have asked is, how critical is the squish band measurement on a Combat head? What happens considering modern fuels when you increase the squish band while lowering compression?