Does anybody have a torque specification for tightening down the top and bottom cover bolts on a 8.5" CC compressor?

We also need to replace a few missing bolts. Does anybody know what grade of bolt is appropriate? There are no grade markings on the originals, but then they're special items with "T" heads and so one wouldn't expect any. As these bolts carry a considerable load, I'm guessing that grade 8 or similar is called for.

This brings up an interesting question. When did bolts of different strengths become commonplace? We like to think of steel as being different grades and strengths now, but a century ago there really were very few grades of steel, mostly going by carbon content. It had always been my understanding that bolts from the depression and earlier were probably pretty close to being a low carbon steel and strength was obtained by using more bolts or increasing their diameter.

I would be interested in hearing what the experts have to say.

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In 2008 Western Railway Museum purchased T-bolts from a manufacturer:

M. B. Fasteners
3310 Moser-Young Road
St. Clements, Ontario, Canada
N0B 2M0
519-699-5433
FAX 519-699-6146

It may be necessary to supply drawings.

M B Fasteners makes tee bolts for WABCO. If you want to order from them the quantity will have to be large enough for them to custom make the bolts. When I ordered the bolts Al referred to above, I ordered 100 or each of four different lengths. I do not know if this order was large enough for them to make the bolts, or if they just felt sorry for me. The tee bolts I ordered were head bolts for the D-4 air compressor. WABCO had already told me they were obsolete and they could not supply them. They were ordered in grade 5 material, not the original grade 2 material.

Many tee bolts need replacement because they have been over torqued and stretched. Once a bolt has been stretched the nut will tighten against the change in pitch without providing adequate clamping force. At WRM we always tightened tee bolts with a torque wrench to avoid over tightening and stretching the bolt. Most modern tee bolts from WABCO are now grade 5.

Regarding Rick's question about bolt grades, I have noticed in the GE literature that they began advertising alloy steel bolts for traction motor axle bearing caps in the early 1920s.

Many thanks to all who have replied.

Since posting I've had a quick stab at an engineering analysis. I may well have got something wrong, but If I haven't, mild steel bolts are indeed more than adequate.

The high pressure steam cylinder is 8.5" diameter and can contain steam at up to 200 psi. Multiplying area by pressure gives a force of 11,349 lbs. From a drawing (I haven't the compressor here at home) I believe that the are 9 bolts around the circumference of the HP steam cylinder. Assuming these bolts share the load equally, each bolt carries a load of 1261 lbs. The bolts are 3/4" -10 NC and have a core diameter of .6255" and a core area of .3073 square inches. Dividing load by area gives a stress of just 4,103 psi, which is well below the proof strength of even a grade 1 bolt (33,000 psi).

So now I feel comfortable using anything that fits.

Torque is trickier because the formula for calculating the torque required to produce a desired preload i) involves a constant that depends upon surface finish and ii) assumes clean well lubricated parts. The torque required to produce a preload of 1261 lbs works out to just 16 ft-lbs. I don't think that this can be taken very seriously, but it does suggest that a torque of 125 ft-lbs (normal for 3/4" bolts) should produce more than enough preload. In principle 125 ft-lbs of torque could produce a stress of 32,500 psi, but this is still below the proof strength, and in any case the of condition of our bolts and nuts is such that I doubt whether we'll get anywhere near this.

So we'll just clean up our threads as much as we can, apply lubrication, and then use 125 ft-lbs.

Curiously there were a couple of locations on our compressor where the T's of what appeared to be original bolts weren't long enough to prevent rotation and a wrench had to be employed. I don't see this as a serious issue, and also don't see that getting the T's just right is important enough to warrant the cost and trouble of either buying in bolts from Backshop Enterprises or having some made. We'll just use off the shelf bolts (perferably with oversize heads) and grind or machine away enough of the heads to make them fit. Anything that just fits shouldn't rotate, and if there isn't enough interference to ensure this, there should also be plenty of room to apply a wrench.