I've just transferred the innards from a low chassis coil engine into a 77xxx crankcase.
Done all the mods to take the bigger crankshaft.
The old crankcase is stamped +1/2, and the new crankcase is stamped -1.
Can't see any markings on the gears. The gears mesh easily, but very little if any discernible clearance. The old setup had done many thousands of miles including a JOGLE. I don't have any other gears to try.
My question is, does the clearance increase or decrease as the engine warms up?

Location: Bonnie Galloway

In theory they will increase in diameter and therefore decrease the clearance, but by how much ??
J

Location: As far east in Kent as you can get

Yes but would the expansion of the crankcase compensate for this to some or all extent? I my experience around 2 thou seems to be reasonable trouble free tolerance when fitting unmatched used gears.

Location: Auckland NZ

I think the aluminium crankcase would expand roughly twice the amount of the cast iron gears.

I would always try for a bit more rather than a bit less backlash- noisier but then the engine is usually quite noisy anyway.

Tony.

Location: Malvern, Melbourne, Australia.

Thanks for the replies.
I've managed to find my ancient set of feeler gauges, and can just get a 2 thou gauge between the teeth, 3 thou gauge long gone, but a 4 thou won't go in, so I will leave it at that!
Interesting conundrum about the heat expansion though.

Location: Bonnie Galloway

Aluminium expands about 13 microinches per degree fahrenheit, iron about half that. The cam and crank centres are about 3" apart and assuming it all runs at about 150 degrees F then the gap will widen about 0.003".
All figures approximate. Say between 2 & 4 thou.
(6.5 x 3 x 150) / 1000000 = 0.0029

Location: Melrose, Scottish Borders

Thanks for that Jim. It's good to know that the clearance will open rather than close as the car warms up.
I've never seen this rather important point mentioned on here before!

I've closed the bottom end up this afternoon, so it's **** or bust now!

Location: Bonnie Galloway

When I first playing with 7's I made the mistake of meshing a pair of gears too tightly, they started to break up on the M3 heading down to my first Beaulieu!

Location: Auckland NZ

On the same subject. Does anyone do new timing gears at the moment? At one time steel ones were available, are they still?

Stuart.

Dave Dye made a set of mag ones for me a couple of years ago. Andy Bird advertises, but he sat on my old ones for a year and then went off the radar completely.

Location: Somerset

Thank you, Dave is doing my supercharger drive at the moment, I will ask him.