why does a 2.5 hybrid pistons not hit the valves

[cj011]

When doing these hybrids, why does the 2.5cc hybrid still stay as a non-interference motor?

What does I have to calculate to find out if my pistons will hit the valves? I know it's a combination of stroke, rod length, and pin height. I am unsure though exactly that I have to figure out.

[speeduphoria]

you need to figure cam lift and piston height in the cyl at tdc

[90TurboVan]

My 2.5 is interference. Head and block were decked just enough to clean them up, Neon MLS headgasket with stock '96 2.0 cams, stock 2.5 pistons. I thought it wouldn't be interference, but when I turned the crank over while setting the timing, pistons started to touch valves (didn't have cams set first).

[cj011]

So how does one determine cam lift if it will hit the pistons or how far past the head does the valves go? I take it piston height will be a relationship between the deck height and stroke, rod length, and pin height of the piston?

Keep in mind this is all new to me so without a specific description or calculations to figure it out, i will be asking away.


thanks guys

[detobias]

even with all the calculations in the world, it's still a good idea to set up your combo with some modeling clay on top of the piston(s) and cycle the engine over by hand (cams hooked up), then pull the head and measure how much the modelling clay got compressed

you could repeat that with 2 pistons at TDC and just cycle the cams to determin if it's 'intereference' or not

[cj011]

detobias, your right. I am just throwing around different combos. I am chasing this so I don't put a combo together that will be bound to fail from the get go from parts hitting

[90TurboVan]

They only hit when the timing is really far off. If you don't break a belt or have it jump like 10 teeth, it'll be fine. Most engines are interference from factory, ours are kind of odd for four cylinders.

The only way I can think of to actually measure it is to get the 1+4 intake and exhaust valves all the way open, set the head on a flat surface, measure how far off the surface the valves hold it. Check how high your pistons are at TDC, distance above/below the deck. Measure your piston dish. Subtract the piston popup distance (positive for piston above deck, negative for below) from the piston dish depth. Add to that the thickness of your headgasket. If that value is greater/equal to the head measurement, you have an interference motor. I think some people were cutting valve reliefs into the pistons in order to make a 16 valve non-interference.

[nemesismachine]

Quote:

Originally Posted by 90TurboVan I think some people were cutting valve reliefs into the pistons in order to make a 16 valve non-interference.

You mean Venolias with the reliefs become more of a non-interference motor? or am I reading that wrong...

N

[lancer402]

my 2.5 hybrid is an interference between the valves and pistons, but the service manual for a 2002 PT says the valves will hit each other if the belt breaks.

so if my belt breaks all hell is going to break loose

andy

[john doe]

Quote:

Originally Posted by lancer402 my 2.5 hybrid is an interference between the valves and pistons, but the service manual for a 2002 PT says the valves will hit each other if the belt breaks. so if my belt breaks all hell is going to break loose andy

Which service manual? All 2.4 DOHC engines are non-interference stock.

[lancer402]

well I thought I read in my 2002 pt service manual that the valves will hit each other. Wrong?


andy

[john doe]

Is that the factory service manual and not a chiltons/haynes? 2.4 cams are smaller then even 2.0 cams and the pistons sit pretty far into the block at TDC. About 0.2" down if memory serves.

[lancer402]

Quote:

Originally Posted by john doe Is that the factory service manual and not a chiltons/haynes? 2.4 cams are smaller then even 2.0 cams and the pistons sit pretty far into the block at TDC. About 0.2" down if memory serves.

mY service manual is the actually chrysler book. I was testing my 2.5 hybrid with 2.0 cams and one exhaust valve and one intake valve installed. I put one valve at a time, all the way open and rotated the piston crank with out the head bolted on. One of them hit the piston, even with good size reliefs in the pistons.
but I cant remember if it was exhaust or intake that hit.

andy

[gkcooper]

The 2.4 is interference valve-to-valve.

[john doe]

Hmm. I'll have to investigate more on this. By valve-to-valve do you mean the valves can touch each other? Because when I was assembling my 2.4 I turned the cams and the crank all independent of each other and never got any binding using the larger 2.0 cams. It was a 97 engine but the changes are very small for the PT and going in favor of non-interference.