Britbike forum Forums Thread Like Summary - 650 crank in pre unit 500

Hello, is anybody able to help please?
I have a pre unit T100 engine, aluminium cylinder, 1956 I think. Larger bearing in timing side case. The original crank is broken but I have what appears to be a serviceable 650 three piece crank and con rods (white metal cap type) 1.125" main shafts stamped 6T on the web. I have read that I can use this 82mm stroke crank in the 500 motor and it looks like it will fit OK but can anybody let me know the problems doing this? I'm assuming I will need to make a 1mm plate between the cylinder and crankcase. Is there a problem with the tappets sitting 1mm lower in the tappet block? Do I need to lengthen the push rods to restore rocker position. Is it necessary to change the crank balance? I will only be using the bike for gentle runs and would like to return it to serviceable condition looking as it was when I last rode it 40+ years ago! If this modification looks reasonable I will split the crank to clean any sludge before assembly. Can anyone point me to what is necessary to realign the crank once cleaned.
Many thanks for any help.

On many p/u timing covers there isn't material enough for a circlip retainer groove. Some still try to fit oil seals and just let them sit there without, which pushes them toward the crank nut. How well it seals then is anybody's guess. I have converted a few back to a bronze bushing, made in a top hat shape to fill the oil-seal space. In my view a good fitting bushing is adequate for the purpose, and it should be remembered that it served Triumphs well for 25 years.
My opinion only, and entirely up to you of course, but it would be very low on my list of priorities.

SR

I can't think of any real problems with this. Don't worry about the push rods, or the followers sitting 1 mm lower in the block. The balance factor will be changed due to the lighter pistons, how much can be calculated, but in any case it will be higher than the 50% quoted for the 650. It might vibrate more at lower rpm and less at higher, but these things are trial and error territory, and I wouldn't hesitate to try. And I'd expect great results too.
The crank pieces are located by precision ground bolts (all these are) into two reamed holes in the cranks and flywheel. Triumph machines these to high standards and close tolerances, and any crank I've cleaned out clocked perfectly after re-assembly.
Just be gentle and meticulous and don't over-torque the high tension 1/4" bolts. If there's a torque number somewhere, I've forgotten it, but somewhere around 10 to 14 Nm or 8 to 10 lbft maybe?

SR

The Triumph workshop manual is of the "sufficient" variety on torque, and you may have difficulty getting a socket/torque wrench on anyway. 10 ftlb would be about right. Have a very close look at the white metal bearings and crankpins, preferably use some engineers blue to make sure they touch evenly. This was often bodged back in the day, as repairs were supposed to be "back to the factory", which most people could not afford, even if they had the time for their ride to work machine. If in doubt, use later shell type rods, they are easy enough to come by (I have at least 8 sets, and that, the later '69 on ones))

It's what I do, on a simple V-block fixture made from wood. I use ball bearings that I've honed out for easy fit and removal.
Add a dial gauge and turn a couple of rings to run it on to avoid the keyways. Simple as it is, you're able to instantly recognize if the crank is straight or bent.

SR

Do a mock up first......Put the crank into the cases, bolt them together, and spin it....

As simple as it is, it works. A bent shaft will immediately show up, even just eyeballing it.

SR

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If the timing side oil feed nose has been biffed about, I'd strongly recommend getting the timing cover modified for the later unit type oil seal, and have the "nose" machined true. The bronze bush feed was never great. Seager Engineering and others can do the mod.

For recovery of dodgy noses

https://www.skf.com/uk/products/ind...ion-seals/wear-sleeves/skf-speedi-sleeve

I hear what you say, I've used speedi-sleeves myself when the nose won't clean up at 0.020" down, but ALL my engines get the oil seal coversion!

Seager weld up the boss if it is thin as part of the service. Like all things, a job has got to be done properly, be it replacing the bush or fitting the seal!

The bush is more reliable than a seal, but has to be an accurate fit on the crank nose, so renewal isn’t so simple.