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How do I save it from total disaster....

When selecting new cams, what would be the minimum valve piston clearances you could have? I'm assembling a 10.3 CR, twin-plug, ITB FI, Tec-3R crank-fired engine based on a 3.0 liter SC engine for a friend. I'm almost finish assembling the engine and discovered a major problem to resolve.

The measured valve to piston clearances: Intake = 10.5 mm Exhaust = 9.0 mm

It was over-looked to inspect and scrutinized the data about the the new cams before assembly. Had I been aware of this problem in the beginning or the lack of experience would save me many man-hours.

Cam timing is set @ 5.6 - 5.8 mm with 0.1 mm lash as recommended by Webcam. And valve lift: Intake = 0.496" (12.6 mm) Exhaust = 0.450" (10.43 mm) as per spec sheet provided by the manufacturer. Inadvertently, I used the cam timing data as the maximum valve travel and thought the clearance was sufficient. Big mistake.

So how do you save this project from total disaster? Comments, suggestion, etc. are welcome and highly appreciated. Thanks.







Tony

Last edited by boyt911sc; 06-28-2012 at 02:52 PM..
Old 06-25-2012, 01:33 PM
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I don't think you have a problem.

From the build it sounds like the pistons have deep valve pockets to provide that much valve clearance.

If the valve clearance is as you say (Intake = 10.5 mm Exhaust = 9.0 mm) at TDC and your cams are set to (5.6 - 5.8 mm) at overlap then you still have nominal 5mm clearance at overlap. The trick is seeing what clearance you have 20 BTDC to 20 deg ATDC. Most important is right before overlap when the piston is chasing the exhaust valve closed.

Remember, Maximum valve extension is tied to maximum cam lift and this should occur when the piston is at the bottom of the stroke and you have (10.5 +70.4mm of clearance) tons of room.
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Old 06-25-2012, 02:02 PM
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btw, put two turns on a valve adjustment screw and see if you can rotate the engine without valve contact.

The pitch on the adjustment screw is 1.0mm/turn.

Does the engine rotate by hand?
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Old 06-25-2012, 02:04 PM
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engine does rotate.
Old 06-25-2012, 05:47 PM
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adjustment screws on valves turned all the way loose.
Old 06-25-2012, 05:49 PM
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Timming high over lap cams?

First time trying to do this. Is there a difference from doing a stock engine? So far we have been told to do 1 and 4 first. Anyone near Norristown Pa have any experience in doing this and want to check our work?
Old 06-25-2012, 05:52 PM
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I am not close but would be happy to come by.

Install #1 rockers (Intake and exhaust) while crank at #1 TDC. Tighten down the lash until it just touches the cam. Rotate engine. Make sure you go 360 degrees. Your issue will be at overlap if you have one. If you can complete two rotations then you need to turn in the adjuster more.

Add 1/2 turn to the valve adjusters (I and E). This will open the valve 0.5mm. You have just reduced your valve clearance by 0.5mm by opening the rocker. Rotate the engine. Did it clear? If so, add another half turn.

Repeat this process until you get to 2.5 turns. If you have 2mm of clearance on I and E sides you are fine.

Looking at how deep the valve pockets are in the top picture you should not have any issues. Again, The max lift of the valve will occur when the piston is far away from the cylinder head.
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72 911T project car. "Minne"
classicautowerks.com - EFI conversion parts
Old 06-26-2012, 08:14 AM
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Found the culprit........

Quote:
Originally Posted by jpnovak View Post
I don't think you have a problem.

From the build it sounds like the pistons have deep valve pockets to provide that much valve clearance.

If the valve clearance is as you say (Intake = 10.5 mm Exhaust = 9.0 mm) at TDC and your cams are set to (5.6 - 5.8 mm) at overlap then you still have nominal 5mm clearance at overlap. The trick is seeing what clearance you have 20 BTDC to 20 deg ATDC. Most important is right before overlap when the piston is chasing the exhaust valve closed.

Remember, Maximum valve extension is tied to maximum cam lift and this should occur when the piston is at the bottom of the stroke and you have (10.5 +70.4mm of clearance) tons of room.
James,

Thanks for your input. I was totally exhausted last night after spending the whole day working on getting the engine finished and committed a big blunder in the installation of the rockers (exhaust). Half of them were binding/stuck and felt like they were on the cams.

The exhaust rockers were removed and re-installed. All were checked and free from binding. The cam timing took longer than expected because of my eagerness to put all the parts. I should have done the #1 and #4 intake only in place during cam timing instead of having everything installed. The racing valve springs were a lot harder than stock valve springs.

I could have easily cranked the engine close to one hundred times yesterday in my effort to investigate if the valves were touching the piston/s or not, cam timing process, and valve adjustment. The good news is that what I thought a major disaster turned to be a good learning experience for me in engine rebuilding.

Tony
Old 06-26-2012, 07:05 PM
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Old 06-26-2012, 07:05 PM
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