M33; running in after a rebore.

[Rog1]

the last time I had a rebore it was a Japanese 2-stroke. The advice then was to watch the tacho; up to half maximum revs for the first 100 miles, and a 1,000 rpm higher for every subsequent 100 miles. Also, don't slog it or rev it. So I have two questions.

1) What's the current thinking on running in?

2) What on earth is a "tacho"?

[Derek996]

The running in advice is probably fine. A "taco" is a revcounter, aka tachometer. You won't find one of those on an M33  Just keep the engine speeds low initially, go easy on the throttle, gradually increasing how hard you run it over the first 500 miles or so.

[Rog1]

 A rev counter?! What will they think of next? Thanks for your advice.

[Rog1]

The bike's back together and I'm running in. I've done 50 miles so far at a maximum speed of 35mph, but I've realised that must be much lower than half maximum revs and I can go a bit faster. But how much faster? I have a July 14th 1955 copy of Motor Cycling with a road test of an M33 outfit, and it gives speeds at RPMs. Maximum speed in top gear is 64mph at 4,700rpm, of course on sidecar gearing.

I have no idea what the redline is for an M33, but maximum power is at 5,500 rpm. On solo gearing that gives me a theoretical 88mph at maximum power; I should be able to run the bike in at up to 44 mph for the first 100 miles. Now, M33s have slightly higher gearing than B33s, and they'll do a theoretical 84 mph at max power in top; probably where this figure, which I've often seen quoted, comes from. I expect this is where the recommended cruising speed of 50-55mph for a B33 comes from, being (at 55mph) two thirds of maximum speed. I remember reading years ago that this was how to work out a British bike's cruising speed.

My M33 is in theory able to cruise a little faster than a B33, but I'll stick to 50-55 for this 70 year old machine.

[neil1964]

Just use mechanical empathy.
Let the engine spin freely up and down through the gears. Do not accelerate hard or use excessive engine braking.
Keep the loads gentle but progressive.
Racer friends of mine laughed at the idea of running in after a rebore, just say warm it up to operating temperature and go for it.
I am more cautious than that and am running in a 'DBD 600' at the moment (I am aided by a rev counter!) letting it spin through the lower 4 gears with light throttle loads up to 4000rpm which is about 65 or so in top on my 21T engine sprocket. .
I'll change oil at 500miles by which time I hope that will have started to explore the upper end of the rev range.

[Rog1]

Thanks for your comments, folks. All run in now, and better than it's ever been. There's a slight increase in vibration now when I'm at 50 or 55 mph, probably due to the slightly heavier piston; it's not unpleasant.

In theory I could have taken it up to 90% of maximum revs during the last 100 miles of running in, but that's about 80 mph so I haven't. Took it gently up to 65 once, just for a few seconds.