Combat Pushrods

Joined
Mar 22, 2007
Messages
59
Hello,
Im currently getting my crankshaft balanced, bronze bushes for the main bearings (crank case) as the interference is gone. As well the oil and breather mod for combat engine and the MKIII camshaft bushes. But the Engine builder asked if the pushrods were the shortened ones. I'm not sure how to accurately measure the length of them as my vernier inst that long. Can you buy shortened pushrods for combat motors?

Regards Bryan
 
Joined
Jan 10, 2005
Messages
1,043
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I must admit I measured mine with a steel rule and lopped off .060". As long as the cut is straight and within .010" it should be OK but it's worth doing the job properly to get good rocker to valve contact. Not sure mine is that clever.
RGM do .060 shortened steel rods on their top end page 3.
http://www.rgmmotors.co.uk/main.htm
 
Joined
Nov 7, 2007
Messages
22
Are you sure it has a Combat head? These were stamped C near to exhaust rocker cover area. I think a Combat head had 40 thousands of an inch skimmed off to raise compression hence the need to shorten the pushrods which Norton forgot to do! If it has RH marks then it is not a high comp. head. All you need do is check against a standard pushrod but 40thou. is not much and is easily accommodated within the adjusters. I race a Combat motor with standard pushrods and have no problems and it doubles as a regular road bike. A lot of rubbish has been talked about Combat motors and remember these motors were trouble at the start of their lives which was 35years ago for gods sake! If you dont constantly redline it through the gears or cruise it at more than 5,000rpm continuously even the breather system works fine.
 
Joined
Apr 7, 2004
Messages
1,691
Sounds like your paying for a real nice job for the motor. That is a good and reasonable question from the builder.
But it all comes to a simple check to put this issue to bed. Set the valve clearance after putting a small a smear of non drying bluing on the end of the adjuster on the face of the contact area. Now run the engine though a cycle. Your looking at the mark on the valve stem tip. It's a wider mark than you might expect. You want to see the mark centered on the tip diameter viewed up and down not side to side. If the smear is off center the push rod length needs changing. If they are left off center the valve guides will take the hit. The height of the jugs comes into play as well as the cam followers and since both of these items will need a check for squaring and truing it's best to take the ten minutes and do the check on the push rods. A new set of stronger push rods and a set of mushroom adjusters also come into the stack up and are worth the trouble IMO But I hit 7000 RPM on most rides.
 
Joined
Mar 22, 2007
Messages
59
Thanks Guys,
I will have a look at the website.
Yes the head has the C stamped on the head. From my understanding it's not just the fact that the head was shaved but also that the cam has higher lobes which sets the rockers at an sub optimal angle. But now I going beyond my depth. The other thing that I didn't mention was that the push rods have some slight scratches. Is this a problem?
 
Joined
Apr 7, 2004
Messages
1,691
Push rods are a fusible link in a over rev situation scratches at the head gasket area is not uncommon, replace them, and fit your gasket at the push rod tube.
 
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