On my 1st generation black powder frame Colt 1873 SA I have excessive barrel/cylinder face gap. At first when checked, thought it was 0.012”, but on closer examination it proved to be 0.015”.
From what I have read and been told, the basic fix is to remove the barrel, face off barrel at thread diameter and recut one or more threads, reinstall the barrel, if required face off the barrel to correct clearance and re-cone.
As this Colt was mfg. in 1886, I do not want to do any permanent modification as that would lower the collector value.
As I understand the physics, upon firing, the primer extrudes to the rear and pressure immediately increases in the cartridge case. As pressure builds, the cartridge case moves to the rear, reseating the primer and the cartridge case expands, conforms to, and grips the chamber walls. As the bullet passes into the chamber throat, the cylinder moves forward with the bullet, stopping when the cylinder base pin bushing gas ring contacts the forward frame face. When the bullet leaves the chamber throat and enters the barrel forcing cone, a rapid change of pressure forces transpires. The pressure now forces the cylinder rapidly to the rear and the cylinder ratchet comes to bear forcefully upon the recoil shield.
The barrel/cylinder face gap at this point is now at its maximum limits.
One can buy or make shims for the cylinder pin bushing to reduce cylinder end shake (AKA destructive battering ram) clearance, this sets the cylinder back, but does nothing to remedy the excessive barrel/cylinder face gap.
I am thinking of making a form fitted recoil shield shim of the correct thickness (0.008”) to reduce the barrel/cylinder face gap to 0.007”. As these are modern solid head cartridge cases, I do not think 0.008” will create a problem with BP pressures. As the shim will be form fitted into recoil shield recess, there will be no movement of it and will be further retained by the base pin. Shim stock can be purchased in precise thickness and of tempered steel. Once the shim fits correctly, a transfer punch of the correct diameter through the front base pin hole can mark for the hole location.
The shim would be form fitted cut as per the masking tape example here.
I am not an engineer, nor stayed at a Holiday Inn last night, so any input and critique as to my approach would be appreciated.
From what I have read and been told, the basic fix is to remove the barrel, face off barrel at thread diameter and recut one or more threads, reinstall the barrel, if required face off the barrel to correct clearance and re-cone.
As this Colt was mfg. in 1886, I do not want to do any permanent modification as that would lower the collector value.
As I understand the physics, upon firing, the primer extrudes to the rear and pressure immediately increases in the cartridge case. As pressure builds, the cartridge case moves to the rear, reseating the primer and the cartridge case expands, conforms to, and grips the chamber walls. As the bullet passes into the chamber throat, the cylinder moves forward with the bullet, stopping when the cylinder base pin bushing gas ring contacts the forward frame face. When the bullet leaves the chamber throat and enters the barrel forcing cone, a rapid change of pressure forces transpires. The pressure now forces the cylinder rapidly to the rear and the cylinder ratchet comes to bear forcefully upon the recoil shield.
The barrel/cylinder face gap at this point is now at its maximum limits.
One can buy or make shims for the cylinder pin bushing to reduce cylinder end shake (AKA destructive battering ram) clearance, this sets the cylinder back, but does nothing to remedy the excessive barrel/cylinder face gap.
I am thinking of making a form fitted recoil shield shim of the correct thickness (0.008”) to reduce the barrel/cylinder face gap to 0.007”. As these are modern solid head cartridge cases, I do not think 0.008” will create a problem with BP pressures. As the shim will be form fitted into recoil shield recess, there will be no movement of it and will be further retained by the base pin. Shim stock can be purchased in precise thickness and of tempered steel. Once the shim fits correctly, a transfer punch of the correct diameter through the front base pin hole can mark for the hole location.
The shim would be form fitted cut as per the masking tape example here.
I am not an engineer, nor stayed at a Holiday Inn last night, so any input and critique as to my approach would be appreciated.