Piston-Valve clearance

[The Black Flash]

I'm installing higher-lift cams in my alfa v6; I've had the valve pockets in the piston machined deeper to suit, but I want to check everything before I put it all together.

I've read a lot which says that you should have about 2mm (0.080") inlet clearance with the piston at TDC. However, the Alfa is an interference engine (hence much woe if a cambelt snaps), so even with the normal cams this won't be the case.

So what do I do? I've got a figure for valve lift at TDC for the new cams (2.55mm). I figure I could set the piston at TDC, wap a bit of clay on the piston, put the head on without a gasket, turn the camshaft until I see 2.55mm of valve lift, then measure the clearance using the clay, and hopefully see a couple of mm or more. Does that sound right, or like the ramblings of an idiot?

A very brief trial run using a rule has shown that I have around 5mm of valve lift before I hit the piston, without a gasket. But I want to be sure!

[one eyed mick]

Not having worked on Alfa's but would guestimate that at 5 mm +max 1'5 for a gasket you should be ok.Is it too much of a job to mount a head with valves for 1 cyl only and place plastercine in valve pockets 3/4 mm thick ,time every thing up and rotate the crank 2 turns by hand ,strip and measure what is in the pockets add the value of a compressed gasket which will give you a fig for clearance . I have used this to check piston to head figs on deisel engines figs were between 1 and 1.1 mm so with a guestimate of 5/6 mil you should be ok but timiming will have to be bang on HTH

[anonymous-user]

It's also an easy job to trial build the head with a set of soft springs (something you can push down with your fingers) install and time your cam, then mount a dial gauge, and swing the engine over (say 20 deg crank either side of tdc non firing) and push down the valve so you can measure the "free space" between valve and piston) Obviously if you have hydraulic lifters you will need to make up a solid "dummy" lifter as appropriate, and it's a good idea if you have cam pulleys with adjustment slots also swing the cam end to end to see what clearance you have at the extremes of the timing adjustment.

[stevieturbo]

Another option is to ask the cam company if that cam is known to cause problems.

[anonymous-user]

I personally wouldn't trust anything/body other than my own measurements! Who knows what combinations of block height, rod length, piston height, head gasket thickness, cylinder head height, cam lift, base circle, lifter lash etc etc, will result in what clearance. measure it properly, and you know ;-)

[stevesingo]

This may help with a method of determining clearance.

http://www.s14.net/forums/showthread.php?t=45025&a...

Steve

edited to add the link I should've posted

Edited by stevesingo on Sunday 11th September 22:39

[The Black Flash]

Cheers all - I was really wondering if I could check the clearances before I built everything up and timed it all in, so I didn't have to strip it all down again after. But it sounds like not, so I'll do it the long way.

[Mr2Mike]

The Black Flash said:

I'm installing higher-lift cams in my alfa v6; I've had the valve pockets in the piston machined deeper to suit, but I want to check everything before I put it all together.

I've read a lot which says that you should have about 2mm (0.080") inlet clearance with the piston at TDC. However, the Alfa is an interference engine (hence much woe if a cambelt snaps), so even with the normal cams this won't be the case.

That doesn't follow, since the inlet valve won't be anywhere near maximum lift until well past TDC. The lift at TDC will be pretty small unless you have some fairly wild cams, probably not much more than 1mm.

[ringram]

Dude get with the program. p2v clearance is likely worst near TDC on the exhaust stroke, because thats where exhaust and intake are open at the same time for scavenging. Usually the intake is larger than the exhaust so its the most likely to hit. But yeah 0.080 min is what Ive heard, things stretch a little and expand with heat and load. You can clay the piston top and rotate the engine a few degree's either side of tdc to make sure you get the worst clearance. It may not be until a few degree's past tdc etc.

[Mr2Mike]

ringram said:

Dude get with the program. p2v clearance is likely worst near TDC on the exhaust stroke, .

"Dude" read my reply, I never suggested for a second that this wasn't the case. The OP is suggesting that since the Alfa is an interference engine that this automatically means the piston valve clearance at TDC is minuscule, but quite simply it isn't relevant.

[stevesingo]

Link added above!