My only piston/valve clearance question

[bbh03]

Hello all:

So I succesfully times and went with the following short-cut method to doing a piston/valve clearance check.

It is important to note that the engine is a 3.0 case with Henry's 9.5:1 JE 98mm pistons and QSC cylinders with a standard 3.0 crank. The cam is 964 profile from John. The heads were machined flat, but not too much material was removed (I don;t know the amount). Also, the deck height was set to approx 1.5mm. The pistons have nice deep valve pockets.

rockers on #1 and #4 intake and exhaust, I set the intake rockers down 2.0mm (2 full turns) and the exhaust down 2.5 mm (2.5 full turns) and roated the crank many times, very slowly at first, then faster (separately for each of the 4 valves). I found no interference.

Also, for each intake valve, I set the engine to around 10 degrees ATDC and screwed it down several more turns before touching piston. I did the same with the same result (sometimes not reaching the piston after 3 or 4 more full turns before backing off with no touching) wiht the exhusts at approx 10 BTDC.

No touching. Is my logic okay here? If so, I'm home free!

Thanks!

[kstylianos]

Home free you are! I'd check the other cylinders as well, but I'm paranoid.

Just curious, why did you decide on 1.5mm deck height?

[bbh03]

its actually 1.4-something. It was recommended to go at 1.25 and with one less shim it was just a tad under that, so stayed on the safe side for the clearance and to keep compression at 9.51 or less (single plug)

[Mark McClure]

I did a similar thing......screwed the adjusters down to effectively give 3mm clearance and turned the engine 5deg at a time. Then pressed down on the valve (only one spring installed) to see how much additional clearance. Worked well and was quickest method.

Mark.....

[bbh03]

Thanks. Glad that method works!

[Rob 930]

I checked piston-to-valve clearance on my 930 that had 98 mm Mahle pistons, modified 930 heads with larger valves, and GT2 Evo cams. The new valves were 52 mm and 42.5 mm (up from 49 and 41.5). My machinist, who suspected I would have clearance problems on the intake side, cut some additional "eyebrow" clearance on the intake valve pockets of each piston in anticipation of this problem. Using modeling clay on the piston crown of cyls 1 and 4, I found I had 0.060 in. clearance, which should be adequate (but is on the marginal side). Here's the interesting part: the point of closest approach was not along the axis of the valve (between the valve face and the piston face) -- it was radial clearance between the edge of the valve and the edge of the newly-enlarged piston pocket. I also checked clearance on a couple of other cylinders by using the valve-adjusting-screw method, and I confirmed that the axial clearance was in excess of 0.080 in, so I didn't go further to find the limit. What this means to me is that with some builds, you should watch for radial clearance too, which can only be found by using some crushable medium like clay. I can imagine that there might be a situation where the valve-adjustment-screw method might indicate plenty of "clearance" when in fact the radial clearance is not adequate.

[ChrisBennet]

With 2 turns (.3mm=.2+.1 lash) clearance you are fine.
I would try to get the deck height closer to 1mm. Steve Weiner once told me to shoot for .035-.040" (.88-1.01mm).
-Chris