ET: Engine (Twin) ET35 to valve guide clearance

[nkt267]

I've just been talking to one of our suffolk members who is recommisioning his Norvin after a very long layup.
He has fitted 105 cams and is wondering what the minimum extra travel in the valve movement there should be at full valve lift.
This is something that i have never had to give any thought to with standard components..
Thoughts please Gentlemen..

 

[timetraveller]

There are two clearances to worry about. The valve head to the piston and the valve stem block (don't have the part numbers here) to the top of the valve guide. I rotate the engine so that the valve is on maximum lift and then, using a suitable lever under the tappet end make sure there is one tenth of an inch clearance. That checks the valve stem clearance.
To check the valve head to piston clearance by far the best way is plasticine on the piston cut out and turn the engine over a few times. This is a pain in the whatsit and requires a lot more dismantling which is inconvenient unless uo are doing that for some other reason. The alternative is to go back to the levering the rocker method and to consider where there is the minimum clearance. Note that the inlet valve is opening some time before TDC and the exhaust some time after. This is where patience comes in and you really need a plot of lift against engine rotation for about one hundred degrees about TDC. You will then see how much lift there is at TDC and you can go to TDC and do the lever trick.. The problem is that in many cases it is not the flat face of the valve head which touches the matching angle on the piston cut its but the very edge of the valve which just touched the edge of the cut out. Enough to make a mark but sometimes not enough to feel. I always cut back edges of those cut outs with a scraper unless it is the lowest compression ratio pistons.

 

[nkt267]

The figures that our member has given me are between 10 thou and 39 thou clearance of the collar to guide at full lift depending on the valve in question..I did say to him that it would be best to strip the engine to check piston/valve clearance,and by the looks of the figures you have quoted then the heads are going to have come off anyway.

 

[davidd]

0.010" seems too little to me, but I am not certain about the limits. I had one of my cams clear the guide during building (it had run on the cam for years) but when running after the rebuild, the collar hit the guide. I had not measured the clearance because it had been clearanced years ago and had worked fine. Something during the rebuild caused the clearance to tighten. It did damage the cam. The dynamic clearance needs to be larger than the static clearance.

To be safe, I need to make new valves with the shoulder on the valve slightly higher. Greg Brillus did this on his twin racer by raising the valve shoulder 0.080", I believe.

I am surprised that the clearance on the Norvin is so small, because the lift of the 105 intake valve lift is modest at 0.020" more than a Mk2 (still below 0.4"), while the ex valve lift is the same or lower than a Mk2. My cam has a 0.48" lift, so the clearance should be there for a much lower lift cam.

David

 

[nkt267]

He is going to borrow my mounts and gauges and check the clearances properly and the i will shorten the top of the guides for him,,IF they have some metal left to remove. I don't know if he had MK2's in before or more normal lift cams..

 

[timetraveller]

David is correct and it is a bit of a can of worms. It not only depends upon the static figures but also how aggressive the cam is and what valve springs are being used and are they still in good order. If the time and interest is there then I would still recommend checking valve head to piston clearance as well. I suppose we should have asked whether the engine is going to be raced or subjected to high revs for some other purpose. If the owner is not going to exceed four thousand revs and the springs are in good order the there is probably nothing to worry about anyway.

 

[Bill Thomas]

If it's got the modified rocker feeds, I would check there is enough swing on the rockers,
I have had to grind a lot off the top of the rockers in the past ?.

 

[Sten Jensen]

Not sure what the clearence should be for a racing engine, but one easy method I used years ago on my street Comet was to tighten the adjusting screws in the rockers to give 1 mm negative clearence, i.e. 1 mm (0.04") lift when the camfollower was at the base circle. This is about one round of the adjusting screws. Then I turned the engine VERY carefully and felt that nothing would touch.
If something do touch you need to find out what and where.

 

[davidd]

Sten,

That is a good solution if your engine is together, but I like the safety of clay!

Bill,

It is an excellent point. I spent an entire day fitting locking rocker feed bolts to the last racer I built. It takes a long time to make all those pieces operate together with a high-lift cam. I would be surprised if the stock locking rocker feed bolt parts would allow the rocker arm to move far enough to have the ET 35 hit the guide.

David

 

[Bill Thomas]

Hello David, I made my own Modified R/Bolts many years ago, I am sure I didn't have to grind the rockers.
But it was a long time ago !.
Don't know why it is needed now ?, Others on the Forum have said they have had to grind the new ones,
I can see a lot of damage for people who don't know about it ?.

Same as T.T. says, I grind the pockets off the pistons, Even 9 to one, I have seen where the edge of the valve rubs the pocket.