Intake valve hits piston (2011)

I'm the ? owner of a 74 850 Commando with a problem. When taking my bike apart for a valve job I noticed it looked like the intake valves had been hitting the piston, there was a burr on the piston crown at the edge of the intake valve pocket. I thought maybe the PO had floated the valves at high RPM and went on with the valve job. To make a long story boring, when I reassembled after the valve job, I find I still have valve interference with the piston. Since I don't know the history of this machine I don't know if the head has been shaved to oblivion or if there is some other problem I need to find and fix. Is this a known problem or do I have an oddball setup?

Check cam timing and spec for sure

Snakepit said:

Check cam timing and spec for sure

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Will do, I should mention that it started and ran fine before the valve job, usually one kick if tickled. That kept me from tinkering with the valve timing.

Amongst all don't forget to check valve clearance to piston! Inlet valve: turn crank to piston at 5 degree after TDC and exhaust valve: piston 5 degree before TDC, tilt over the rocker with a pry bar and measure the clearance until the valve hits the piston. You must aim for at least 0.06"

I'd suspect the cam timing first too but also look for evidence a piece of carb slide
didn't pass though in the past. It valve sticking on return in guide that could do it too. Maybe just too tall a piston and just grinding clearance would fix it forever but consider grinding similar off other side.

Most 850 pistons have flat tops. If the pistons have valve pockets it may mean that the head or deck has been milled, a Combat or similar high-lift cam has been installed, high-compression pistons are in there or perhaps some combination of the three.

If the valve seats have been replaced it's possible that the valve head "sits proud" and protrudes farther into the combustion chamber than it should when seated and when fully open. When the mania was upon us for installing hardened valve seats I actually had the two intake valves somehow kiss each other because they both protruded too far into the chamber.

Good luck--and let us know how you get on



Tim Kraakevik
kraakevik@voyager.net

You may have big valves in it. Also, if your pistons are flat top, but protrude noticably above the barrels at TDC, then you have high compression pistons. As Hobot says, grind the edge of the pockets to make clearance. Also, as Nortonspeed says, you should have .06" clearance under the valve.

Thanks, good advice from all. Just to make sure I understand, at full open I should still have .06 remaining movement before the valve touches the piston? That's at the specified degrees before and after TDC for the valves.

A 74 850 should have flat top pistons. Somebody before you made modifications...

maylar said:

A 74 850 should have flat top pistons. Somebody before you made modifications...

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This one has flat top pistons, but there has been a small cut made in each one opposite the intake valve. If the intent was to clear the valve, it didn't work.

Early tests show 0 clearance at TDC for the inlet valves.

I assembled mine with some plastercene in the valve pocket to test clearance & rotated engine. Also made a cutter from an old valve, assembled head without gasket & cut them slightly deeper.

Wow, mine have pockets nowhere near as deep as those. Also, my pistons don't reach above the cylinders.

Combats were the only factory model pistons are .050" proud of cylinder at TDC.
All C'do's use same rods so its new pistons not pocket cut enough and/or the barrel has been cut down. Looks like someone put in hi CR pistons as well as 2S cam profile to approach Combat pull.

Sounds like you have a high lift cam. Your pushrods should have been cut down. You can add a cylinder base gasket. They are available in various thicknesses. Some atlas valves are too long for commandos also so could be a part number error and incorrect valves..

hobot said:

Combats were the only factory model pistons are .050" proud of cylinder at TDC.
All C'do's use same rods so its new pistons not pocket cut enough and/or the barrel has been cut down. Looks like someone put in hi CR pistons as well as 2S cam profile to approach Combat pull.

Click to expand...

I don't think so, Steve. As I recall, Combats used standard pistons. Only the Production Racer had different pistons. Combats got the compression increase by milling the head, not the cylinders, so the pistons shouldn't be any higher above the deck than standard Commandos.

Ken

Ken I'm sticking to my story until more Combat experienced feedback refutes my statement. I was surprised to find this and quizzed Norton world on it a decade ago. Only Combats came from factory with pistons .050" above deck @ TDC. I was told to check this to check if crank throws aligned together for same proudness.
If rods and cranks all the same and the cylinders not milled, which they ain't, it must be the pistons difference. Milling head don't raise pistons just CR. If I've got this data wrong I must re-think my reality and memory competence to even leave home.

Duh I forgot to ask, does JIms rods/piston stick up above barrel like the Costwroth's?
iIRC you milled head for Norton rods and Costworths.

hobot
PS, they just repaved my road course to work same as Barbers track, in Heat Wave so totally melded to the surface and densely compacted in deep dark asphault and matrix, so smooth it can't be felt, while I was in Catskill's. To a water skiier its like going from choppy lake > up river to mirror smooth surface you can carve up like a surgical ice skater or razor edges of sure grip. When first hard rain hits it will increase traction by cavitation cavities to a quantum crystalline texture that merges with tire compound into the brain stem...

Steve, in 1972, all Commandos, standard and combat, intially used what Norton called an "Interim" piston design, part numbers 063338 and 063339. The only difference from the previous standard pistons, part numbers 062459 and 062462, was deeper valve notches in the piston to accomodate the combat cam lift profile . The interim pistons were installed starting with engine number 200976. These pistons still had the slots in the oil ring grooves. Starting with engine number 204166, Norton installed the "final 1972" piston design, part numbers 063348 and 063349, in all Commandos, standard and combat. These pistons also had the larger valve notches, but had solid skirts. They were used for all the 750 Commandos from then on. Both the interim and final designs were for a nominal 9.0 compression ratio, had the same deck height, and were considered interchangeable by the factory. The combat got it's higher compression by milling the head 1 mm/.040". The combat engines did not have higher compression pistons from the factory, and they should have the same deck height as standard Commandos. All this info is from the Norton factory service releases.

Ken

Ugh, right off the top of your head too. Will have to save that to files for easy reference.
Ok then do other Commando's pistons stick up above the barrel like my 2 Combats? If not why not? . I'm confused again.

As one who was a member of the engineering staff at the time, I can state quite categorically that road-going production pistons, either 750 or 850, did not protrude from the top of the cylinder. I've built, and rebuilt, both enough times to be absolutely certain of that!
Any engine with pistons above the deck either have shaved barrels or non-standard pistons.