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Here is the answer to what is OHC interference

shoestring

Well-known member
Joined
Jun 20, 2009
Location
Swampscott, 01907
OK, measured this on an untouched B230 head with a correct base measurement of 146.1mm. That's the distance from the deck to the valve cover surface.

The intake valve is 9.80mm below the head deck, the exhaust is 9.58mm below. A stock head gasket is 1.2mm compressed. That means that if you have a stock engine, and your piston is EXACTLY zero deck, the M cam's exhaust lift of 10.5mm is about as far as you can go. Therefore, the A cam (10.5mm lift int/ex) is going to be ok, but the B cam (10.6mm int/ex) may not be, depending on piston position. Yeah, I know, 0.1mm is very little, but you've got to draw a line somewhere. If you mill the head, whatever you take off REDUCES the maximum lift you can run and still be non-interference.

Anything with 11.0mm or more is definitely interference.

So there you go. Moral of the story is change your timing belt. They couldn't have made it easier.
 
I think the bigger variable is the block height from Volvo.
Connecting rods and pistons are easy to accurately make, block heigh it one that a lot of manufactures seem to have gone “good enough!”
 
I think the bigger variable is the block height from Volvo.
Connecting rods and pistons are easy to accurately make, block heigh it one that a lot of manufactures seem to have gone ?good enough!?

+1 to this.

Without climate control, thorough process controls, and other advanced monitoring methods it will be the most difficult for a manufacturer to nail down a routine block height. c-c lengths on rods and pistons are fairly easy to control in comparison.
 
I had my machinist refer to a 530 head as "sophisticated European hardware" once. His delivery was so dry it wasn't until I'd left that I realized he'd been busting my chops.
 
Ha!

And yes, just to spell it out in yet another way… Almost, it not all, all modern engines are interference. So, if you really want to get every last dollar out of your $17 timing belt, stick to a lame engine. Otherwise, be smart and make more power at the same time by modernizing your engine with an interference setup(yes, there’s more to it than that).
 
Yah but most modern engines use chains now, people wreck the car or other parts deteriorate long before that chain breaks
 
Yah but most modern engines use chains now, people wreck the car or other parts deteriorate long before that chain breaks

Chains might not break as easily but they do stretch. It's because on modern engines they often use 1 narrow simplex chain. It's cheaper to make, keeps rotating mass acceptably low and as a consequence keeps mileage reasonable.
But many modern VAG, Mini, BMW and PSA engines (and probably more) suffer from cam chain stretch.
It's also common on motorcycle engines.
 
one of the old dudes, maybe stealth or JVL said it 12mm but lots of variables. I thought it was about that when I checked with a dial and straight edge years back on an unmilled head.

I got some b21 flat top máhles from rpr back in the day and they had way shorter comp height than stock. I had to deck the block a bunch to set it back at zero.
 
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