Wednesday, 12 June 2013

Starting on the engine!

Well, summer's here and the bike is coming together. I've finished my welding work, the wheels are still away, and most importantly I have some time away from work to get to grips with things at home.

So, in a break from repainting the kitchen we will start work on the bottom end.
 
I'm dithering around polishing the crankcases which is a noisy & messy job & which stops me rebuilding the engine. Someone also suggested that polish would hinder cooling - but equally it is obvious that the engine was once polished - it is deeply tarnished now, but you can see it is still very smooth - you can also where it wasn't polished!
 


But, I have all the parts and can get moving with the rods & cranks.
 
You'll recall from previous posts that the rods have been cleaned and polished and have spent the winter protected by some pipe insulation in their own box. I have small end bushes and big end shells from Drags along with new big end nuts, also from Drags.
 
These are shown in the adjacent picture and will have to be pressed into place and reamed to the finished size.

 
The sizes are shown in the table below and are around 11/16" - but will have to be sized to fit the factory specification using a suitable adjustable reamer. In this case we will use a 21/32" - 23/32" reamer, which are readily available but are of highly variable quality. Beg, steal or borrow a decent one if you can! If you must buy an internet cheapie, take a look at a few things before you use it and if it is no good, send it back!

Take the reamer to pieces. Measure the length of each blade, and make sure they are equal.
  1. Look at the retaining nuts, make sure they are not too loose and clean the threads
  2. Clean out the grooves in the reamer body and make sure the blades can sit snuggly down in the grooves. You don't want any swarf in there.
  3. Put the reamer back together, and measure the diameter across two opposite cutting edges (this only works on six blade reamers) at each end of the blade. The dimensions should be the same! They are not supposed to be tapered. Repeat the operation for the two other pairs of blades.
So, to fit the bushes. You can use a press for this, if you have access to one, but a bench vice comes a close second. You could also use the draw-bolt method, but I suggest the loads are probably too high for that approach. These bushes are quite tight.

  1. Find a socket or a piece of tube which is small enough to support the small end eye, but large enough, both in length and internal diameter, to receive the old bush.
  2. Open the vice enough to receive the rod, the socket and the new bush plus a couple of millimeters.
  3. Find a piece of soft sheet (brass for example) to protect the end of the new bush.
  4. Align the socket, the rod eye and the bush. You might benefit by putting a short rod of about 1/2" OD inside the assembly, just until you have the new bush a couple of millimeters into the rod, just to keep things lined up.
  5. Nip the whole lot up in the vice with your protective sheet on the jaw pressing on the new bush.
  6. Operate the vice to push in the new bush, pushing the old one out into the socket. You will probably find the initial force required is much higher - and you might get a bit of a bang as the bush starts to move. This is normal. Don't be afraid to apply a bit of heat to the small end eye to get it moving - but go easy, this is aluminium. No Oxy-acetylene here!
  7. Carry on pushing until the new bush is more or less flush with the face of the rod. My bushes are about 0.5 mm over length at the moment.

The next job is to drill the oil hole. Do this before reaming and you don't need to worry about deburring later. The Square Four has 3/16" holes in the small end eye.

Next - reaming to size. Get ready with your gudgeon pin, your internal micrometer or bore gauge & a 1" micrometer.
 
Now, adjustable reamers are really scraping devices. They are designed to scrape out small quantities of metal from holes and as you have seen (from when you inspected your new reamer) they are not as rigid as solid reamers and can only be used to take small cuts.

When you set the reamer up for the first cut, back off the adjustment collars (i.e. screw them away from the square end of the reamer) to reduce the diameter of the reamer until you can put the reamer into the bush. Now adjust it larger until it just begins to touch the inside of the bush. Turn it using a tap wrench a couple of times with some oil.

Be careful to twist the reamer such that it is concentric with the bush; make sure the blades pass right through the bush. That way you will minimise the potential to cut a tapered hole.

You want to increase the size of the hole in very small steps, testing with the gudgeon pin at each stage -  DON'T SKIMP ON THE CHECKING!! Small end bushes are relatively cheap, but replacing them repetitively is tiresome. Increase the size of the cut by turning the collars by 1/8 of a turn each time, and turning the reamer in the hole 3 or 4 times. Always make sure that the top collar is tightened down.

 

A few more thoughts:
  • NEVER turn the reamer BACKWARDS – can You could break or chip a blade.
  • Never put more than ¼ turn on the collars
  • If the reamer jams, loosen the blades and pull the reamer out and start again. Remember a jam will leave a number of ridges and these have to be cut away or it will happen again.
  • Make certain that the collars are tight.
  • Clean the reamer after each pass.
  • Do not push down on the reamer too hard or it will jam. Take it slow and easy - this is supposed to be a hobby remember? You don't need to hurry, you are enjoying yourself right?
  • On each turn try to stop in a different place.
  • Use oodles of cutting oil.
So, now we have our reconditioned connecting rod:


So, repeat the operation for the other rods and you will end up with a little heap of scrap like this:

So here is a table showing the dimensions I have ended up with:

Small End Bush
Big End
Factory Specification
0.6868”
0.6863”

0.003" wear allowable
1.375” min (journal)
Cylinder 1
0.686”
1.3765”
Cylinder 2
0.6869”
1.3745”
Cylinder 3
0.6863”
1.377”
Cylinder 4
0.686”
1.3775”

I had to order a couple more bushes from Drag's since I wasn't happy with No. 2 & No. 3. The table above shows the new dimensions.

And now, over the last couple of days I have polished the cases. I made sure that I polished only the areas that were polished in the factory all those years ago:

Next stop, bottom end assembly!



No comments:

Post a Comment