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Checking Piston to Valve Clearance

I would like to check the piston to valve clearance in a manner other than the “turn in the adjuster screw” method; I’m getting some odd readings doing that. I’m getting 2+mm on the intake, 1.5+ on the exhaust. The latter being a little tight, according to Wayne’s book. (Engine is 911SC, 9.8:1 Mahler, stock cams. Ringer was that case was line-bored at Ollie’s, changing a lot of things.)

I know it will involve some disassembly and retiming, but I think I need to do this for peace on mind later on. Any good web sites or instructional videos out there?
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Old 01-01-2018, 05:34 AM
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If ollies did the case they should have included thicker base gaskets. Did you measure deck height? I think Waynes book is a little conservative on the clearance recommendations.
Old 01-01-2018, 08:32 AM
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Yes they did include thicker base gaskets but they were inadequate, even stacked. With them deck height was too high. I had to get 1mm gaskets to get the specified 1mm. That was some time ago.

But even with that I have inadequate piston to valve clearance on the exhaust side. So I need to measure that and probably add another base gasket to get that right.
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Old 01-01-2018, 03:12 PM
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Those are stock Mahle SC pistons?

I would double check your deck height and double check your cam timing. It is possible to adjust your cam timing somewhat while still staying with in spec. That might be something to try.

Additionally if your heads seating surfaces were machined, that could make the valves a little closer.

I think you are correct in being concerned about the exhaust to piston clearance. The exhaust valves are the ones that will be the first to hit the pistons on an over rev.
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Old 01-01-2018, 03:55 PM
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Valve to piston clearance @ Z1.........

Quote:
Originally Posted by Steam Driver View Post
I would like to check the piston to valve clearance in a manner other than the “turn in the adjuster screw” method; I’m getting some odd readings doing that. I’m getting 2+mm on the intake, 1.5+ on the exhaust. The latter being a little tight, according to Wayne’s book. (Engine is 911SC, 9.8:1 Mahler, stock cams. Ringer was that case was line-bored at Ollie’s, changing a lot of things.)

I know it will involve some disassembly and retiming, but I think I need to do this for peace on mind later on. Any good web sites or instructional videos out there?


Steam driver,

You can measure the actual valve to piston clearance @ TDC (Z1) for cylinders #1 & #4 by using P7I/P7E tools and a dial indicator. With the rockers, valve retainers, valve springs removed, you could conveniently measure the valve to piston clearance using a dial indicator. After the test, install the parts back and perform valve adjustments for the said cylinders. The cam timing will remain unchanged or stay the same as before.

Tony
Old 01-01-2018, 08:22 PM
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Yes, pistons are stock Mahle, 9.8 CR. As far as I know heads were not messed with. Head work was done by them also, but I don’t recall that being done.

Question: how do I correct this issue? Thicker base gaskets? This would also increase deck height, but there is room for that since it is set at minimum (1 mm) now.
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Old 01-02-2018, 04:18 AM
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Is there a number stamped on the head to indicate they had the seating surface machined? Have you considered CCing the heads to confirm your actual CR? That might be worth considering. If your CR is higher than you are assuming it only makes sense to increase the thickness of the base gasket.

Perhaps clay may be a way to confirm both the deck height and piston to valve clearance.
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Old 01-02-2018, 08:23 AM
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I dug out my receipt from Ollie’s on the heads. Yes, they did reface the heads, .25mm. Which is just about what I’m missing on the piston-to-valve clearance.

So, how to correct. Deck height is set to bare minimum, so I guess I could add a .25 shim to each cylinder base. They already have a 1.0 shim to account for the case machining.
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Old 01-02-2018, 03:06 PM
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if you adjust your cam timing to the lower end of the range, that might get you what you want to see, easier then taking the engine back down to the case.
Old 01-02-2018, 03:35 PM
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FWIW, I used JE 9.5 pistons and my CR ended up at 9.8 to 1 with a 1mm deck and my heads have not been machined.

You might want to CC your heads and do a compression ratio calculation just to see where you are. I don't think you want to go over 10 to 1? You might find that with the extra .25 base gasket your CR is still 9.8 and your piston to valve clearance is greater. Worth a check IMO.
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Old 01-02-2018, 04:06 PM
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Cr

You have to be careful adding shims, its better and less vol.removed when cutting the piston pockets, on a 95mm bore the CR change is .25 point of compression with a .010" shim, I have also found that the pockets are not always exactly the same angle as the valve, also you can use a transfer punch or sharpen an old valve and with the piston at TDC, head in place locate the center of your valve as the guide would put it in relation to the pocket with a small punch mark,then sweep the pocket to see how the valve is located, they are not always centered either.
Mike Bruns
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Old 01-17-2018, 10:41 AM
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Did you read this. It was posted sometime ago in another thread by Neil Harvey of Performance Developments.



There is a very simple way of checking the clearance between the valves and the piston.

You need a degree wheel fitted and zero’ed out to true TDC on the crankshaft. Then a simple indicator and holder. Make sure the indicator is steady and has no movement other than its measuring travel.

Fit 1 piston to the engine along with the cylinder. Fit any base shims etc you think you want to use. At this time you can cc the piston and the head chamber to calculate the engines compression.

Turn the crankshaft so it is at 20° BTDC. You will be measuring from 20° BTDC to 20° ATDC every 5 or 10°. This will depend upon how much data you require.

Now fit one head to the engine with both valves fitted but without springs and the rest of the parts. With the engine at 20° BTDC , measure each valve from its seated position on the valve seat to where it touches the piston. This is done by hand with the indicator. Make sure the travel of the indicator is aligned with the valve stems. Record the distance travels by the valve. Also make sure the valve goes into the pocket if there is one without it touching the pocket edges. Otherwise this will give you false readings.

Now move the crank to 15° BTDC or 10° if you want less data. Repeat this until you get to 20° ATDC.

Now if you have the valve lifts your cam produces from your cam supplier at these same degree positions, do some simple math to obtain the clearances. You may find its negative but its on paper and easy to calculate the amount each pocket needs to be machined. But remember your CR will change as well.

If you don’t have the valve lifts of your cam, remove the piston and rod from the engine. Re fit the head with checking springs along with the cam deck and camshaft, timed as you expect to run it. Now re fit the indicator on the back of the rocker adjuster. Make sure it is seated well and cannot move as the valve is depressed. Now repeat the same measuring positions from 20° BTDC to 20° ATDC. This will give you the valve lift numbers that you need to subtract from the valve seat to piston numbers.

This will give you the P/V clearances and this way, if you are fitting new pistons that need the pockets machined but do not know by how much, this method will give you any interference on paper only.

If you wish to side step this last part, you can send us the cams you wish to use and the cam timing you expect to run and we can run the cams and supply you with the lift numbers at that timed position.
Last edited by Neil Harvey; 01-03-2018 at 10:22 PM..
Old 01-03-2018, 10:18 PM
Old 01-17-2018, 11:17 AM
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P to V

Yes I did, we have done the same process to find the crank position where the valve is closest to the piston on a given camshaft, on a lot of the grinds I use it is 7 deg. BTDC on the exhaust and 10 deg. ATDC for intake, when I set it up with light springs on Cyl 1 and no valves in Cyl 3, cams timed for the first check for V to P on #1, I mark #3 piston with a transfer punch or old valve stem machined to a point to find center of pocket. You can put the valve in the collet on the mill with the piston in a piston vice and see exactly how it looks or simply sweep the pocket with a caliper set to valve diameter.
Mike Bruns
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Last edited by MBruns; 01-25-2018 at 03:24 AM..
Old 01-17-2018, 04:58 PM
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