Combat engine - rebuild options

Hello Jamie, you say there ss camshaft in, Is the breather outlet on the left hand side like a dommie ,If so at the end of you camshaft there is timed breather plate and spring in the middle of your camshft there will be four holes ,these are there to let the engine breathe, So any tampering with drilling hole in the crankcasesto let the engine breathe better will upset the whole oiling system. My advice is to leave well alone and run your engine as it is with good 20/50 oil like Millers or Morris's and do not worry about your superblend bearings these were made for racing, So 18k is nothing ,and don't install a tap or an antisumping valve in the oil lines , just make sure your oil pump is in good condition ? Yours Anna J Dixon (IE,Bs,Eng)

If it is a Combat, the camshaft driven breather will not feature on this model. The Combat head was skimmed 0.040" most Combats were de-tuned after the initial problems by the company so lower compression pistons may have been fitted. The cam is probably a 2S and not a SS. If the compression is up around 10:1 a 1mm ( 0.040")base plate should be adequate to reduce the compression. You need to find out what pistons are in there. (part no. etc) If you have the large separator and breather on the rear of the crankcase, you could modify to breath through the timing cover, See the picture in the most recent Roadholder. If The superblends are in good condition leave them alone. You say the engine is in good condition, if its not knocking or smoking, leave it. Concentrate on accurate ignition timing and balanced carbs to get it running nicely.

Yours Dave Evans

Hi Jaime,

Having done the breather mod 12 years ago when restoring my combat engine, my recommendation is leave the stock breather alone...it works just fine. Moving the oil pickup might be required if you plan to run constant high revs, but for the normal riding that most of us do, the stock setup is just fine. I'm still running my engine with high compression, however lately the fuels here in California are causing a pinging (pinking) problem. I'm planning to purchase some race gas and blend it with pump gas to see if that doesn't fix this.

David

NCNOC

Previously wrote:

Just currently fully restoring a Combat engined roadster and need some advice on options. The engine is in good condition and Superblends had already been fitted - using the original ss cam and standard compression ratio. The question is, shall I just replace the superblends for new bearings and maybe fit a 2mm base gasket and that will be fine OR should I go down the route of having the crank case's modded for improved breathing and move the breather position. The bike will only be used on the road. The bike has covered around 18,000 miles already just on Superblends so I am tempted (and so is the bank manager) just to go for option one.

Appreciate your thoughts

Jamie

On the additional base spacer I use an 030" with no gasket. RGM supplied it and it reduces compression to about 9.3:1. I wouldn't go any lower. I did originally use a gasket either side and the performance drop was noticeable with approx 8.7:1 but that was with a std cam installed in my replacement cases.

I trust that everyone talking about fitting a thick base gasket will not forget the effect this has on the rocker gear. The rocker arm angles were not a strong point in the Commando cylinder heads to start with and adding anything like a2mm thick gasket is not going to help matters.

Adding a meaty head gasket to the mix compounds this problem and it would be no surprise to find a big drop ina bike'sperformance with both fitted.

Finally, to really throw stones in the China shop. Drillinglots ofextra holes in the timing case and adding one-way breathers to the venting system tends to solve one problem but create lots of others.The original camshaft timed breather worked in conjunction with the movement of the pistons so that oil laden air, in the crankcase, was deliberatelypushed around the bottom of the engine to help lubricate all the moving parts. This was known as the 'bellows' effect and asthis airentered the timing cover italso helped to push surplus oil, in the cover, through twooilpassagesback to the sump.

Now it is dead easy to mess thissystem upresulting inthe pistons descending and all the crankcase air disappearing up a one-way valve instead of lubricating canshaft lobes, mainbearings etc. Worse still. As the pistons rise, and this valve shuts, the engine crankcase then experiences negative pressure, which does indeed stop oil leaks from the joints but...............causes the cam followers to be sucked down hard on the camshaft lobes wearing them out double quick or sooner plus aircan also sucked in through the crankshaft oil seal. Which in turn causes a vacuum in the primary case which then leads to oil being sucked from the gearbox.

Hands up if you keep getting too much oil on your clutch? Now you know why.

Agree on rocker angles, especially exhaust. However, I don't think he really wants a 2mm gasket in there anyway and certainly not one of those squishable 2mm aluminum headgaskets a well known Norton dealer was trying to sell on ebay a few months ago!!

On the timed breatherI would not run them together but he has a Combat so won't have the camversion. Interesting statement on the cam followers, wouldn't have thought of that. Are you saying this affects all one way valve set-ups?Is the vacuum created so high that it applys similar or even morepressure to the tappets than open cam pressures of around 200lbs? I cannot believe that. I have just replaced my 22000 mile std RGM cam (presumably a Newman grind) with a PW3 and there is little wear on the cam and tappets. However I have used Amsoil synthetic over that period except for running in rings. I know I shoulduse straight 40 or 50 but the proofetc.!! Also have a belt drive andhave not experienced oil in the chaincases from the box but I follow your thinking. I think much depends on the valve and there are US experts who state the one way breather valve must be as close to the cases as possible to work effectively. Mine is adjacent to the battery so maybe it is not as effective as it could be.

Back in 72 I recall my 69 Fastbackleaked like a sieve but since I didn't have it long enough to sort out I never did get to the bottom of the problem but I don't think the cam breather worked too well.