Tight Tappets

I have just taken my Pots of with the head attached due to a blown base gasket and i have put it all back on and made sure the push rods are aligned at the rocker arms and have proceeded to do the tappets gap and have noticed that the exhaust tappets are very tight and cant get the 25mm gap that is specified in the book for the combat, hell i cant even get 20mm but the inlet gaps are fine what is the problem here.

25mm that is around an INCH???

I hope you mean 0.25mm?
(0.010" = ten thou.)

Ar you shure the clearances were correct prior to taking the head and barrel off? The original combat motors had shorter pushrods in order to restore the rocker geometry with the higher compression head
(ie skimmed). So if by some chance this motor was rebuilt with new pushrods they may be too long. The other possibility is that your replacement base gasket was quite a bit thinner than the one you removed thus moving the cam followers closer to the cam shaft and reducing the push rod clearance. If none of the above applies why not run the engine for 5 minutes with this clearance and then recheck and see if it has settled in a bit,

woops yeah i mean .25mm 10thou.

The gasket i replaced it with a High Temp pressure resistent gasket silicon as advised in my topic about the blown gasket, so therefore it would be thinner then the original gasket and when it had the gasket the gap was good but very close to the rocker arm itself.

Would removing the rocker arm and grining a bit off it were it goes onto the top of the Valve work or should i just replace the Silicon with a New gasket.

What is the best way to go about removing the rocker arms and spindles without removing the head, if i was to go down the grinding track.

I wouldn't recommend grinding the rockers. For a quick fix you could try to get a set of the socket head rocker adjusters which may possibly give you a bit more to play with, alternatively you could take it apart and refit a base gasket. It does however sound as though you have push rods that are too long which will effect the rocker geometry with consequences on valve lift and side pressure on the valve guides. A new set the correct length can be purchased from most comprehensive Norton parts supply companies. Specify the combat type (I'll bet L.A.B. who is a veritable font of knowledge probably has the appropriate specifications to hand).

Combat heads were shaved .040 inch so the pushrods should be .040 inch shorter. You can shorten your existing pushrods by placing them in the freezer for a while, then pull off one of the steel ends. Shorten the aluminum tube the appropriate amount and push the end back on. Some Combats were shipped from the factory without shortened pushrods.
Unless you had the head shaved, it seems unlikely this is your problem. Since you did not remove the head from the cylinders, the clearance should only have been reduced by the thickness of the gasket you removed. Are you certain that the pushrods are seated in the lifters?

I am a little apprehensive that you used a silicone gasket compound. I hope you were very diligent that a very thin film was used so none was forced out and inside the crankcase. This will cure into little balls and find its way into the oiling system. I have a connecting rod I keep as a reminder. The silicone found its way into the end of the crank and starved the rod bearings. The rod turned black and split vertically from the big end to the small end! Ruined the rod, crank and crankcases. That is why I use only the anaerobic sealers. They do not harden where the excess is squeezed out and it dissolves in the oil and is harmlessly removed on the next oil change. Where the mating surfaces meet, it cures.

I used LOCTITE HI-TEMP RED RTV SILICONE Gasket Maker, im not sure if it is anaerobic, maybe you can tell me. When i had the head done it was shaven along with the pots to ensure a perfectly flat surface, this may have caused a reduce in the clearence and the thin gasket has just made it that much smaller. I might do as you said and shorten my pushrods. If the gasket maker i used is'nt anaerobic can you tell me one that i can use.

I can't imagine that skimming the faces could lead to a total inability to get tappet clearance. The rocker angles could be all wrong or the springs could coil-bind on full lift but there should still be enough thread on the adjusters.

I have never taken the head and barrel off of a 750 in one piece but, As Ron L implies, contrary to what the books say, the pushrods can do other than positively locate in the cups on the followers. They can sit alongside and if you're really unlucky, catch two followers at once.

I wonder if, with the cylinder removed, the followers can drop low enough before reaching the retaining plates, to allow this to happen.

My advice would be DO NOT START THE ENGINE but first back the adjusters right off and fish around at the pushrods to see if they can locate more positively at the bottom.

If this doesn't work, I think you're going to have to pull the head off again and check everything. It might even be a good moment to check valve to piston clearances and that the springs are not coil binding.

RTV silicone is NOT anaerobic sealant. The anaerobic sealants are acrylates, not silicone. I use Loctite 518 Gasket Eliminator. There is a high-temp version, Loctite 510 also. Hylomar Gasket 2000 is another brand.

79x100 said:

I can't imagine that skimming the faces could lead to a total inability to get tappet clearance.

Click to expand...

Well Skimming the faces plus it being a Combat head and may well possibly have the wrong length pushrods and a thinner gasket this would make it around .040" to .060" difference wich is enough to upset the tappet clearence.

It is not that there is not enough thread on the adjusters it is that the pushrods are to long and are causing the rockerarm itself to sit ontop of the valve. The valves go up and down but the i cant get clearence on the exhaust tappets as the arm sits on the valve.

Ron L i will take your advice and go buy a Anaerobic Sealant as i asked for the Loctite 518 which they didnt have so they gave me the RTV

I have looked at few old paper gaskets the crushed thickness is .010. I made steel base gaskets years ago much better. Kenny Dreer had these made at one time. You may find one listed under decompression base gasket on old brits or british bike connection sites.

I am going to take the head of and shorten my pushrods about .060" - .070" and then use an anaerobic gasket sealer, this should cure my problem and get rid of the tappet noise i had aswell.

dave M said:

Specify the combat type (I'll bet L.A.B. who is a veritable font of knowledge probably has the appropriate specifications to hand).

Click to expand...

The standard pushrods are advertised on the BSA Regal website as:

= "06.7940 PUSHROD (INLET) 8.15" NM 25495"

= "06.7941 PUSHROD(EXHAUST) 7.3" NM 25497"

And these appear to be the types now available for all year models (although of course they can be shortened and these are likely to be available from certain parts suppliers).

Technical data from Norton Factory Service Release N3/ 73 850 Model Commando "Stage One" High Performance Modifications technical data gives the following info:

= "Push rod length assembled (inlet) 8.130"/8.166""

= "Push rod length assembled (exh'st) 7.285"/7.321""

There is information on how to shorten the push rods (by 0.037" [0.95mm]) to suit the stage one skimmed (by 0.075" [1.91mm]) cylinder head, recommending that an equal amount of metal be removed from both ends of the push rod in order that the end caps re-seat properly, and not partially rest on the tapered part of the rod.

The NOC Service Notes mention that pushrods may need to be shortened by 0.040" for Combat spec. engines, or for other skimmed heads: "Take as much off the pushrods as you did off the standard head"

I have also found there can be insufficient clearance if 'mushroom' type adjusters are used.

I have got my head off and have placed the pushrods in the freezer how exactly am i supposed to get the hardend ends of the rods so i can shorten them.

OK i got them off the pushrods, shortened them about 1.5mm (just took of the beveled edge and made a new one), Used the loctite 518 gasket eliminator and have put the pots back on, i will get the Head gasket knealed on the weekend and the old girl should be back on the road by sunday/monday. Thanks you to everyone who has helped me out here your info and advice is appreciated. :lol: :wink:

If you would like to check your mod. To see if you hit the nail on the head. Get to the store and buy a small tube of non drying blueing. Smeer a few drops on a thin piece of paper. Now with the valve at it's clearance point slip the inky paper in there as to butter up the adjuster side. Take out the paper.
Now bump over the motor through one cycle. The mark made by the adjuster on the tip of the valve should be slightly off center left to right but dead center up and down. If the mark is off center up and down you need to adjust your push rod length. wipe off the excess ink and try another.

Just a thought; would the higher lift cams (like a 2S) create a similar problem with pushrod length too?

Scooter

No the back side would be the same. But anyone fitting hot cams should do this check.

tappets

Dave M.

"Ar you shure the clearances were correct prior to taking the head and barrel off? The original combat motors had shorter pushrods in order to restore the rocker geometry with the higher compression head"

Actually one of the mistakes Norton made with the combat was that they didn't think to shorten the pushrods. This is commented in the Norton Service Notes published by the Norton Owners Club in England. The poor rocker angle is what I attribute to premature exhaust guide wear on my '72 combat engine.

I have yet to get the head back on due to the head gasket needing to be kneeled and not owning an oxy i cant do it my self, When i get the head on i will do the check with the ink.

Illf8ed your right they never did come with shorter pushrods as i asked norvil and got the reply

Hi
Combat uses std length. Use short pushrods off 0679501 set
regards
Kate