Britbike forum Forums Thread Like Summary - Crankcase Vent

Hello all,

I’m in the middle of rebuilding the engine for my T140. This engine was completely stock when I took it apart and I have no intentions of making major performance enhancements to it, aside from 9.5:1 compression, spitfire cam, and a 3 piece roller bearing.

I’m wondering what people would do with the crankcase vent and primary. Is it advantageous to add the crank seal and separate the primary oil from the engine oil like the older engines were? Or should I just leave it alone and put it back the way it came apart. I really have no compelling reason to change things but now is the time to rearrange the breather and add the seal if I want to.

Thanks

Thrasher +1
The timed breather even struggles with a 650 at sustained high rpm, though this only manifests as sweating at all the engine joints. In moderate use it isn’t a problem at all.

I don’t think this system was ever tried on a T140, as the chaincase breather arrangement was adopted 2 years or so before the 750s appeared.

However, many pre-70 650s have had 750 conversions put on them, but I haven’t any direct knowledge of how they cope with the timed breather.
It may be reasonable to suspect they might struggle even more than the 650 at sustained high rpm. (Stan Shenton in his 1972 tuning book was advising adapting pre-70 cases to the later system, for racing conditions).

If you were to blank off the small holes and install an oil seal, you would have to provide an alternative crankcase breather. One I think may be the best uses the front engine mount chamber. Holes are drilled through to the crankcase and a reed valve plumbed in the top.
Others have installed a vent on the timing cover, next to the inlet cam.

Either of these is possibly more trouble than you want to go to. As said, the T140 system works fine.

With my pre-unit 650 I am content to put up with the original timed breather and the minor sweating when used at sustained high rpm (which only happens on a motorway, say constant 4500+ rpm for hours) in exchange for the independence of the chaincase in terms of oil type and level (for best clutch performance).

...A more considerate reply than I expected, TT! For the most part, I stay off the big roads. There is one highway, between here and my place of employment, which I will take if I'm in a hurry, and I'll do 70 mph, which on my A65 is 4100 rpm. But that's only 15 miles, and I'm not doing "4500+ rpm for hours". For more leisurely riding, there are plenty of secondary highways and even smaller roads around here to take me wherever I want to go.

I find my A65 runs best in the 3500 to 4000 rpm range, smoothly and with plenty of throttle response. This is about 3/8 throttle with my 32mm Miks, and the bike will run all day like that without breathing hard. It will pull easily from 40 mph in 4th gear if I'm not climbing a grade, without lugging or pinking. The only time I've ever had the bike over 70 or over 1/2 throttle at speed is when I was testing for main jet size. And the engine has never seen 7000 rpm in its lifetime. But the opinion has been expressed many times on the forum that it's not good to run an A65 that slowly, which led me to wonder if there's something different about my A65, or if it's simply a matter of preference. If it's the latter, I guess I'm content to merely "bimble about".

I've been tempted to try a 47 tooth on the Trident to keep the revs down.
Nah, that would never work, I'd just go faster.

Yes, no disagreement, bad choice of words.....I had Franz&Grubb, a Triumph shop, do the heads on my double engine Triumph. He measured 69.5 cc on the 1969 dated heads.I'm sure about the volume on the 750 head, did it twice and I have done car heads....I had made a mistake earlier on the cylinder height, .032 is correct...Somehow I did come out with 9.2. compression..

A compression test meter does not measure compression ratio!

Here's info from a respected source on dynamic compression, thermal and volumetric efficiency and brake mean effective pressure...
Think you can apply this to a Triumph?


http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm

For BMEP, My 650 Triumph was on a dyno measuring rear wheel power. True torque from an eddy current dyno, not an inertia dyno , was 41 ft lbs @ 5100 rpm....let's add 10 % for driveline losses to calculate engine torque, so 45.1. Plug in the formula and the BMEP is 170 psi.....