Main Bearings

Cutting the Main Bearing Slots

These can be filed by hand, I prefer to cut them with a round ended milling cutter.

The slots are 11mm wide and I have a 10mm and a 12mm round ended milling cutters !

So using the 10mm cutter, I milled down one side then milled the other side the same amount until I achieve an 11mm wide lot in each of the main bearing's seats.

The cast iron castings provided by Stuart are top quality and a pleasure to work with, very few hard spots (these occur if the cast iron has been cooled too quickly)

Machining the Brass Main Bearings

The main bearings come as a a brass extrusion and again are of excellent quality and easy to machine.

The centre bearing needs to be split and has a locating pin that holds the lower part of the bearing in place.

I recommend drilling the location hole right through the bearing and into the base, to give a starting hole that you can drill deeper into the base as required.

This ensures that the bearing and the fixing pin line up.

It is difficult to measure the depth of a hole that small, so I fitted a slightly longer pin, then ground it down to size, slightly lower than the bearing shell, using a Dremel.

Using a "Top Hat" Tap Assister

One of the problems that many experience is getting tapped holes square to the item they are tapping.

One way around this problem is to Make a "Top Hat" tapping assister, easy to make and easy to use.

The Top Hat shown in the image was made from a seat belt set screw with the head turned flat and the body drilled to accommodate the size of tap you are using.

The tap being used here is a 7 BA - a small tap so great care needs to be exercised when tapping, gently turn the tap a turn the slightly back to clear the swarf then another half turn and so on.

The backward turn is important as it clears the cutting edge for the next bite.

Once the tap is well started the Top Hat can be removed and the tapping can continue.

If the tap thread is bigger than the shank of the tap, drill the hole in the Top Hat a clearance fit for the size of tap in use.

Tapping for The Main Side Bearings

When I drilled the holes for the Main Side Bearings, I drilled right through the casting.

The reason: decreases the chances of breaking a tap and it allows the swarf from tapping to drop out.

I will be using Loctite thread lock to locate the studs in their holes.

Tapping for the Centre Bearing

drill the locating holes in the brass centre bearing first (Tapping size 2mm) then split the bearing with a slitting saw, you can use a hacksaw to do this.

After grinding the locating pin, place the lower part of the Centre bearing in place

locate the Crankshaft in the side bearings and tighten them.

Locate the top part of the Centre Bearing over the crankshaft line up your tapping drill and drill into the base, just enough to clearly define the hole.

Remove the Centre Bearing and drill the stud hole.

Repeat the operation for the other side.

Then using the "Top Hat" carefully tap the stud holes.

Centre Bearing Tapped

The crankshaft and base assembly are taken apart and cleaned ready for the Centre bearing.

Enlarging the Bearing Holes

The Centre Bearing holes are enlarged slightly over clearance size, to allow for adjustment (3mm).

Always use a vice when drilling brass Never hold a brass component with your fingers, it will snatch and spin, slicing your fingers in the process

When enlarging holes on the mill use the feed wheel rather than the hand levers, it makes for a cleaner hole and reduces the likelihood of snatching.

Main Bearing Assembly

Clean all of the part thoroughly debur any sharp edges and assemble the parts.

Locate the side bearings on the crankshaft offer the assembly up to the base, don't tighten the nuts at this stage.

Turn the crankshaft to check that it turns freely if not - check that the bearing are fitted to the correct side as there are bound to be variations.

Add the centre bearing without tightening the nuts, turn the crankshaft to check it turns freely.

Tighten all the nuts, and if you are extremely lucky the crankshaft will turn freely and you will be a very pleased person.

As the chances of the crankshaft turning freely at this stage are pretty unlikely, adjustments will be required.

Adjustments and setting up

It is most important to set up your crankshaft main bearings to run freely, it has a profound effect on the overall running of the engine.

Please Note: It took me some hours to get my crankshaft to spin freely .

Assemble the bearings without the crankshaft in place and try passing a shaft of the same diameter through, this will give you and idea where things are a bit tight.

Use a 7mm reamer and pass this through the bearings, gently easing away any extra metal that might be causing things to bind.

Re-assemble the whole unit and test again.

The centre bearing has a major effect on the setting up of the main bearings and is the most difficult to adjust.

Suggest removing the centre bearing completely - then assemble the two outside bearings, now adjust these so that the crankshaft spins freely.

It can be useful to enlarge the stud holes in each of these bearings a little at a time.

You might need to gently file the internal sides of the bearings to ease them.

Push and pull the bearings just to check there is enough free movement and there is no binding.

It might be necessary to remove metal from the main bearing frame underneath the curved part of the bearing, don't worry if you go a little deep, the bearing relies on the top plate and the studs.

When you have bedded the two side bearings, it's time to tackle the centre bearing.

For this bearing the depth of the main bearing frame is important as it holds the lower part of the split bearing against the crankshaft.

If you have inserted the location pin for the lower part of the bearing, temporarily remove the pin and place the lower bearing part into the bearing frame.

Reassemble the crankshaft and the two outside bearings check that the crankshaft spins freely and that the centre bearing base is a snug fit to the frame and the crankshaft.

You can use thin card under the bearing to pack it up if it is too low.

Once you have the lower half of the split bearing nicely bedded in.

Place the top half of the split bearing and secure it on the studs.

Check that the crankshaft spins freely.

Two methods of adjusting the top half of the split bearing are to, enlarge the stud hole and use card packing under the fixing plates.

As I stated at the beginning this can be a long winded process, so take your time and plenty of breaks.

When you have the crankshaft turning freely, make a note of how you assembled the bearings, mark the bearings on their underside so you know which bearing goes to which part of the main bearing base.

The card packing pieces can be measured for thickness and replaced by thin brass or aluminium strip - you can use card packing, it does appear to last OK, personally I feel happier making brass spacers.

Aligning the bearings correctly will allow your engine to work at maximum efficiency, and add to the life of the engine and bearings.

If you do have problems, and things don't go quite as planned, you can obtain new castings and parts as single items from Stuart, I decided to re-do the Connecting Rods, so a quick link on-line and they were in the post.