Threads, nuts, bolts and tightening

Threads

Austin Sevens were made in the days when British Standard Fine—BSF and British Standard Whitworth— BSW were the norm. Getting new nuts and bolts as well as taps and dies is still possible but not as easy as current thread forms, especially BSW. Most fixings factors can still obtain nuts and bolts but you may have to buy in reasonably large quantities. Alternatively there are a number of small companies specialising in fixings for older cars though you probably pay slightly more this way.

 

I’ve noticed that some of the new  nuts and bolts although of the correct thread have metric sized hex’s that can result in having to use a different range of spanners, especially if old nuts are mixed with new bolts etc. As long as the threads aren’t stretched or the hex mutilated, it’s worth reusing old ones suitably cleaned up.

 

Another thing to watch out for on A7’s is that not all the threads are BSF or BSW. There are some with a 5/8”x16 form such as the starter dog and half shaft. There’s also some odd fine threads on the shock absorber pins and radius arm ends. It’s one of life's great mysteries why these are different, don’t try to recut them with a BSF die.

 

Tightening

Use of a torque wrench is not something often seen with Austin Seven’s. That’s surprising as it is the smaller threads prevalent on the A7 that are prone to over tightening. I would recommend that a controlled tighten is performed on at least the head studs and the big end nuts. Make sure with an aluminium head you’ve got some good thick washers under the nuts to spread the load into the head. Start off by hand and gently tighten, pulling the spanner with just the end of your little finger, then torque in the correct sequence to 10 ft.lb, 15, then 20 and 20 again. Let things settle into place for a while and then 20 again. Keep doing this until none of the nuts move further. Note that over tightening doesn’t help make a better seal, it just stretches the studs making them more liable to fail and locally distorts the head around the studs rather than pulling it down squarely. Here’s the tightening sequence;

Home page | Links to other sites | SPEEDEX History | SPEEDEX Parts | SPEEDEX’s and their owners | SPEEDEX advertising etc from the ‘50s & ‘60s | Where are they now? | For Sale and Wants | Stupendous 750 | Special Builders Guide

The nuts should be followed up once the engine has been run up to working temperature. Try to do this hot for a cast iron head but for an aluminium one let it fully cool and retighten.

 

One thing to watch out for with the later 1937—39 cast iron head. The two front studs (6 & 13) also retain the aluminium water outlet. A number of years ago rubber gaskets were available for this like the side water branch ones. The two studs have to pull down the head, they can’t do that with a soft gasket in the sandwich. You end up either pulling it down hard enough but extruding the gasket or not tightening it enough. Only use a very thin paper gasket with some of your favourite gasket sealant (Hylomar in my case).

Locking

Split pins with castle nuts are a good way of locking nuts and bolts. However if you are torque tightening the nut, invariably the slots won’t line up with the hole in the bolt. DON’T tighten or slacken it to make it line up, try different thickness washers under the nut. While I’m on about split pins, don’t fit ones that are too long, snip them down so that they only stick out about 1/8” and then lightly open out one of the legs to about 30 degrees. That’s all that’s needed and you’ll stand a chance of getting it out again in a reusable state. Wrapping the untrimmed long legs all round the nut looks a mess, its difficult to get them out again and is no more secure than the right way. If you are struggling with an old split pin, trim off the bits sticking out of the nut and shear the rest by forcing the nut off. The bit inside the bolt can be pushed out with a small drift.

Split pinning the big end nuts isn’t a good idea, bits of the legs can break off and end up in the oil and jets. Use Nyloc self locking nuts correctly tightened. Locking fluids or adhesives like Loctite are an effective way of keeping things together. For simplicity, two nuts tightened against each other is also hard to beat.

Sealing studs etc

Head and manifold studs are badly designed in that the tapped holes in the block aren’t ‘blind’ and are drilled right through into the water jacket. Just screwing the studs in and bottoming them in the thread will be secure but  some of them will leak water once the engine is filled up. The earlier three piece rear axles have the same fault around the banjo case. Try sealing the thread, after de greasing, with a Loctite or Green Hermatite.

If you’ve got the studs out make sure that the threads are OK, it’s a bit sad on a rebuild to have to take the head off again to replace a dodgy stud (I know—I was that sad person!). They should screw in by hand most, but not all of the way and not wobble sideways to any noticeable extent. The last few turns should need a little mechanical help with either two nuts locked together or a stud extractor. If one is wobbly, try a different stud or a known to be good bolt thread. If it’s the thread in the block that’s wobbly the best solution is to retap it the next size up and use a stepped stud. Don’t try Helicoils or similar rethreading devices in this application—it makes sealing them to keep the water in very hard.

Another good practice to adopt for studs is to countersink the holes, the threaded one in the block and the head. Gaskets rarely slide smoothly over the studs, usually there’s one or two that the gasket edges up against raising an edge. Countersink about 1/16” around each hole and it will accommodate the raised up edge of the gasket without holding the head off the block. Also when fitting the gasket, they rarely drop into place, don’t try and work it down onto the block with your fingers, it’s easy to bend or distort it. Enter it over the studs and then use the head to push everything into place in one flat movement.

Big ends

Who hasn’t found a stretched big end bolt on a stripped down engine? Always closely inspect the threads for stretching, if a new nut won’t smoothly go down the thread consider using another. Note that three bearing big ends use bolts with flats down the sides to clear the bearing shells. I always use Nyloc nuts for the big ends tightened gradually to 25 ft.lb. If you do want to use the standard castle nuts don’t under or over tighten to get the slot and hole to line up for the split pin. If you must use split pins (why? - they’re a pain!) assemble and tighten the nuts. Note which nut goes with which bolt on which big end position. For those that line up—great! For those that don’t, try another nut, you’ll eventually find a combination that lines up at the right tension. If you’re really stuck try filing the nuts mating face so it rotates further—make sure its flat and square.

Drilling out and Helicoils

Damaged threads in the crankcase and other aluminium components are not unusual (don’t be heavy handed when tightening!!). Damage comes usually from a broken stud that has had to be drilled out. If you have to do this don’t try an EasyOut thread extractor, if the stud wasn’t going to come out the normal way, expanding it and twisting the remains won’t work either—believe me I’ve tried! If you have a broken stud there’s a few hints. One I’ve never tried is to ‘eat’ out the stud with concentrated Nitric acid. It sounds a bit dodgy but apparently the acid eats steel but not aluminium. Take lots of safety precautions and try it on a scrap piece first before your chosen crankcase and clean off well afterwards. I can imagine building a little dam of some acid resistant putty around the stud to hold the acid and also having something underneath to catch it when it eats its way right through, a rag is probably good enough—but have something. And for goodness sake be careful, goggles if not a full face mask are a minimum.

 

If you haven’t got that option you’re into drilling. If the stud is snapped above the flange face, file it down flush and pick up the centre with a good heavy centre punch dot. Usually they snap about one thread down, these are harder as there’s usually a raggy surface that is hard to centre onto. Use the centre punch to push the raggy bits to one side so you can get a good centre mark.

 

Using a small drill, 1/8” or 3mm say, lightly start the hole but only go down a fraction. Check you’re still on centre and haven’t wandered—it’s easily done. Correct by angling the drill in the right direction, keep checking, and then straightening up. Drill through and then go up in very gradual drill sizes until you can see the root of the threads down the hole. Try and pick out the remains of the thread with a sharp scriba, this is easier with bigger threads than small ones as you’ve got a bigger hole to work in! It should come out as a continuous spring. Invariably you’re slightly over to one side. If it won’t come out try putting down the correct size tap, often the drilling and local heat has weakened the grip between the threads, the remains pick up on the tap and you screw the whole lot out of the bottom of the hole. Clean up the thread with a tap and check how good a fit a new stud is. Some damage is OK as long as most of the thread is there and tight. Sometimes you can pick up good thread further down the hole—studs often have an end with a longer thread on or can be cut slightly longer with a die.