rcf925

Piston to Valve Clearance

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So I'm pretty much finished with suspension and brakes on my 68 and now it's on to motor. I'm getting ready to install head and was wondering about piston to valve clearance. Its an E12 head going on a 74 block with 9.5  Piano top pistons. According to my measurements I think .020 has been skimmed off head and none off the block. I'm running IE .292 new cam (not Re-grind)  Should I be worried about piston to valve clearance or will I be ok. I guess I need to know if I should put together with clay in cylinder and pull apart again to check or should I be worried at all about clearance.

piano top piston.jpg

E12 Head.jpg

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+1 with clay it. Also check to see if you need the adjustable cam timing gear, to advance the cam since the head has been machined a bit. The adjustment may help with valve clearance issues, and my not need a thicker head gasket. I measure everything I when building engines. Not knowing clearances can be catastrophic and expensive..



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A safe min of 1.15 to 1.5mm on each intake and exhaust valve to piston clearance, If you are using new or double valve springs. There are other factors to consider like your valve adjustment setting, cam timing and head gaskets. Also if you miss shift and spin it too high. The closer the clearance the more likely you’ll bend valves. Are the valves oversized.? Things to think about. All need to be measured accurately.


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Looks more like an e21head to me.  Could be just the camera angle.

 

but what do I know. 

 

Mac. 

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100% clay it!

And maybe even start thinking about an adjustable cam gear.

My head was just short of final thickness, with a schrick 292, Mahle 9.5pistons, and thicker head gasket.

In order to get proper clearance it took machining the faces of the exhaust valves and advancing the cam timing to compensate for the reduced head thickness.

abca47b57e6de4e2d4e670b32c2da554.jpg
c67039e330588d541d8b4a39646783cd.jpg


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This is going to sound dumb....but hear me out:

 

if they're genuine BMW / original / Mahle pistons, make SURE that the arrows on the tops of the pistons all point toward the FRONT of the engine. I once replaced a blown head gasket on a customer's car (he had owned and driven the car for 3-5 years and the previous owner drove it for a decade before that) and I didn't go crazy cleaning off all the carbon on the pistons for this particular job. I re-installed the cleaned up / resurfaced head, and 3 days later it blew a head gasket again - this time some piston damage was visible. 

 

Once I had the car back at the shop - after thoroughly cleaning and inspecting the pistons, I found that a previous builder (one that was well known in the 80s and must have done a hundred of these) had installed all the pistons BACKWARDS. 

 

Proof that even professionals screw up. First the builder and then me. As they say: "Familiarity breeds contempt".

 

ALWAYS double check everything and never trust the last person that worked on it...even if it's yourself! :D 

 

This was about 20 years ago. I'd like to think I'm smarter now. 

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It's one of those 'if you have to ask' things- since you asked, now ya gotta do it.

 

t

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8 hours ago, mac said:

Looks more like an e21head to me.  Could be just the camera angle.

 

but what do I know. 

 

Mac. 

 

If you see the full size pic of the head, it has E12 stamped on it.

 

E21 looks similar, but with the head chamber rotated 180 degrees.

 

And yes, clay it.  It's easy enough to do

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I'm waiting on dowels for block to put head on, But yes I just bought some clay and will do that as I don't want to ruin expensive build. My nephew put lower end together quite a while ago so I'm probably going to pull pan too. He says he studded mains and motor is balanced but since I got this project in pieces it's a little slow going but I'll get there. 

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