Interference Engine?

[Rosebud]

Q: Is the A+ motor an interference engine?  In other words, is it possible for the valves to come into contact with the pistons? I have 10:1 compression, a fast road cam, 1.5 rockers and a stock crank profile. If my timing chain breaks or I experience valve float, will my valves hit the pistons?

[MiniDave]

Shouldn't, depending on how big of lumps are on the cam

[Rosebud]

Shouldn't, depending on how big of lumps are on the cam

When I posted my question re: valve vs. piston, it was purely hypothetical. Who would've known that 2 weeks later I would answer my own question. The answer is yes.
http://www.restorationmini.com/forum/index.php?topic=2079.0

[94touring]

Which cam?

[Rosebud]

Which cam?

The cam is aa Elgin 103 w/ 1.5 rockers.

[94touring]

Info on the cam.  Basically it's a kent 276.  Nothing too crazy but this gives info with the 1.5 rockers:

Slot drive Elgin cam, specially developed with more exhaust lift and duration for engines 1300cc or larger. Duration is 265 degree intake, 278 degree exhaust. Lift at cam = .265 intake, .293 exhaust. With 1.5 rockers this equates to .399 intake lift and .439 exhaust lift at the valve. Lobe center 108 degrees.

[Rosebud]

Info on the cam.  Basically it's a kent 276...

So, given those numbers, would I have been able to deduce that the valves would interfere with the pistons?

[MPlayle]

If the depth of the combustion chamber at the exhaust valve plus any recess of the piston in the bore while at TDC is less than the 0.439 calculated value, then you will have interference contact.

[94touring]

And if that's the case, you may want to consider some 1.4 rockers from Calver.  Not sure what the math works out to be, but may be better anyways just purely from a street car perspective, unless you are wanting more 7k+ rpm race motor feel.  I know on the evo cam we used on the inno build is similar to a 276/this elgin and it was recommended to stick to a 1.3, so we did. A standard 1.3 rocker is more like 1.2, so we used S rockers to get a true 1.3.

[Rosebud]

Last August I asked if the A+ powerplant was an interference engine; would the valves contact the pistons if the timing chain broke? I had this happen on a V-8 MBZ motor once and it makes a big mess—heads, valves, rocker assembly, push rods, pistons and bottom end, all trashed. My question regarding the A+ was hypothetical. By an unfortunate coincidence, a few weeks later I had answered my own question. The answer is yes.

It wasn't the timing chain that let go, it was a head gasket. Thankfully the damage wasn't nearly as severe as with the MBZ. Watch the video and see if anyone can guess how a blown head gasket would lead to a valve hitting a piston. Or, read the article for the backstory.

Video: https://www.youtube.com/watch?v=Nz28jIrrses&feature=youtu.be&fbclid=IwAR15Loqmvaw3E93rj6Bn2xY7v74f2s8b1z-l236mA2uAvf_zoVk9zpEOqUY

Article: https://www.facebook.com/notes/poser-motorsports/blown-head-gasket-made-worse/3168054043267250/