Crankcase breathing

The Dominator engine was designed with a number of oil mist feed holes and drain channels. If you look inside the Timing Side there are holes all over it to help drain the sump and lubricate the pinions, gears and chains. These rely on what is called the 'bellows effect' to work. ie. the movement of the pistons pushing and pulling oil or vapour through these various openings. 

Serious changes to the engine breathing can upset this arrangement by pushing too much of the crankcase gases out of the engine before they have helped with lubricating parts such as the Timing Gear, Camshafts Lobes, Bushes and Cam Followers. 

Carefully research what you want to achieve and the means of doing this. Otherwise you may end up doing more harm than good.  See photo.

Hi John......The standard Dominator breathing arrangement worked well for the 500cc & 600cc engines. However, the bigger engines struggled especially the Atlas which would try and breath through the engine drive side oil seal. 

When the 650 engine first appeared, the Bracebridge engineers tried fitting an extra breather at the top end which fed surplus gases through the inlet rocker box cap. (Warning....this arrangement does not work well with a pressure oil feed).  Dunstall did something similar but using the exhaust rocker covers. I purchased a set of his vented exhaust covers and this worked for me for over 30 years.

The other version of extra venting that I tried was a PCV valve off the Timing Cover. This used the same breather connection elbow as fitted to the Atlas and was positioned opposite the electronic ignition unit sprocket.  Both set-ups allowing the Bellows Effect to be maintained within the motor.

See Attachment......

Very informative post Philip, thanks. Have attached a photo of my extra breather on the timing chest cover on my 1967 650SS, it has a PCV valve on the vertical section of hose that exits near the real number plate

Can you fit the later oil tank (with the froth tower) on the earlier bikes?

Norton did warn against over filling the oil tank, but if you've had the bike that long you probably recognise that.  My 88SS (1963) has the timed breather on the end of the cam and the usual breather pipe straight out from the rear upper left of the crankcase and doesn't throw oil anywhere.

When these engines were designed, oils did not have anti frothing agents so the volume could nearly double when used hard.  But that shouldn't be nearly such a big issue today.

Blasting these cylinder heads is best avoided for this very reason but often they are so cruddy that there’s no other easy solution. You can buy a special thin paint specifically for sealing porous casting called Glyptal although  it’s quite expensive for a small amount but I have heard good reports of it from friends. After blasting my head, I sprayed it inside and out with VHT silver and baked it in an oven as per instructions. You can leave the engine to do the baking if you can’t use the oven (it does stink the house out) but the paint finish will unavoidablely get grubby marks in it when reassembling as the paint won’t be hard.

Best regards, Al.

It's worth reading up the archive articles in Roadholder mag. Read what John Hudson + others had to say on the subject.

Have followed most of the advice with my 650ss. "Mayonnaise " problem from the oil filler area now minimal after a 60mile ride. I use 20w50 at moment, engine mainly standard [not tuned] with b/e shell oil holes blocked, std larger 650 oil pump, with 3 start worm drive set. Rocker feed taken from oil return with mod adaptor from small twins, fitted.

Rich mixture is a cause of additional condensation in engine. Carefully set carb up but don't go too lean mixture.

Advised to stay with the 36" total length of breather hose [as original to chainguard] as high as poss', tried a pcv, may try it again, not much difference. Catch tank at back of bike, blocked the vent hole in the new oil filler cap [don't overtighten]. 

Keep seeing folk asking about oil level. Change the oil, measure 4 1/2 Imperial Pints. Mark a screwdriver with level from filler neck. Job done. Easy to decide if oil needs to be drained from crankcase after not using bike for some time. 

Work in progress.

Hope this helps.

Somewhen between when my Dommie was made in 1959 and in 1964/65 when I was doing its first rebuild they changed the timing of the camshaft breather by using a different rotary plate.  I discovered this when I bought a new plate. Someone with better knowledge and more patience than me may know what significance it has to the "exhaust" action through the engine breather pipe.  I assume there must have been a good reason to change it.

If you put "Dominator crankcase breather plate" (or similar) into the NOC search box there's an interesting in-depth technical discussion with pic's of the various breather plates, tracing its history.

Hope that helps

Since re-routing my breather into the oil tank and increasing the tank breather to 3/8 " I see nothing at all. The only issue is a bit of mayonase in the tank cap and neck. . I will try a bit of insulation in the tank area to encourage warm up. And perhaps add another tank breather.

Insulate the oil tank Robert? I still recall my days at L Stevens, the west London Velocette dealers, with the Thruxton models coming as standard with a heat shield on the side of the oil tank to prevent the rider burning his right leg (once he'd learned the Dark Art of actually being able to kick-start it in the first place!)

Yes insulate!, most of the time I am doodling along in convoy with club members on green back roads , the tank rarely gets hot enough to drive off condensation. I avoid motorways like the plague , too many berks in cars not concentrating, A roads full of traffic in the South East  and cameras everywhere.