Up to the Boilermaker's Nameplate

In the construction of old, generally coal-fired boilers for use in heavy applications one would generally find above the boiler door a large brass plate proudly bearing the name and location of its manufacturer.

Before the door would stand strong men intent on pushing their shovels into the boiler as far as they would go, depositing material therein as deep and as fast as they could go without killing themselves first.

The boiler itself was designed specifically to take this kind of punishment, day after day, year after year, and, while not being especially pleasing to look at it, would be guaranteed to raise a glass amongst those men who had over the years sweatily laboured at its threshold.

But I digress.

Its another way of saying your cock was RIGHT in a chick as far as it would go and then some.

Hey Mel, how did it go with Teresa?
Pretty good. The lobster was barely cold when I was up to the Boilermaker's Nameplate.

Hey Mike, I hear your wife gives out.
No shit. Every night I'm up to the Boilermaker's Nameplate.