I’m curious to the installation of D style flexible stay bolts specifically in terms of the torque specifications. i.e. when installing should the flexible stay bolt be brought up snug to the socket and turned ¼ more or is snug sufficient ? Any tricks or pit falls to installing multiple rows of flexible stay bolts ?

Ideally, each staybolt should take an equal amount of load. Part of the problem is that the bolts are being installed in a boiler that is not in service so it is essentially a slightly different size and shape than it will be under steam.

Information gathered over the years indicates that different roads and manufacturers had varying requirements for setting flexis. The most common seems to be the practice of backing off bolts a slight amount (1/8th turn, 1/4 turn, etc.). This may be due to the fact that when driven, a bolt will shorten slightly - provided it is a wrought iron bolt. Steel bolts rarely shorten when driven. There has been speculation that the backing off was to allow the bolts to be tight in service when the firebox expands. This is a bit iffy in my opinion. Depending upon the location of the bolt and its angle relative to the direction of expansion of the firebox sheets, it would be difficult to predict with any consistency what these bolts will do.

As a rule, our practice has been to tighten bolts very tight, back them off, retighten very tight, back them off and then set them lightly against the seat. The reason for making them tight initially is to ensure that they are contacting the seat, and to knock down any slight irregularities on the ball or the seat, so that when the bolt is finally set there is some assurance that there is good seat contact. Typically we will tune the bolts with a hammer to ensure that the bolt is taking a similar load as its neighbors.

Tuning is a bit crude, but if you hammer an adjacent bolt to one being installed, you can hear a difference in sound if the new bolt is overtightened to the point of slightly deflecting the sheet and removing the load from the bolt being hammered.

Care must be taken to ensure that the tapped hole is in line with the flexi sleeve to the point that the bolt does not bear hard on the side of the sleeve. If this occurs it is difficult to determine exactly when the bolt seats, and it may not seat correctly. If the bolt is not on its seat there is a good chance the adjacent bolts will be overstressed and more prone to breakage.

Unfortunately there is no magic method. It isn't rocket science but it requires a bit of seat-of-the-pants common sense. Where you want to end up is with all the bolts being equally loaded (based upon the area each is intended to support) when the locomotive is in service.

I hope this helps a little bit.