Del70

are the 70 olds 350 engines free wheeling? thanks

tru-blue 442

??? Not sure what you mean?
Are you talking about a clutch fan?

Octania

I think he is inquiring as to an interference design

If the timing chain breaks, do the pistons hit the valves and destroy the engine?

If that is the question, I believe the answer is no, the engine will stop w/o much damage. But, I have never tried that. You'd have to assume that at least one valve is entirely open, and with a hot rod cam, that might be a lot more open than with a factory cam...

MDchanic

I believe it is Yes.

1989... 1970 Cutlass convertible, 350/4bbl/10.25:1, 103,000 hard miles: Pulled into a parking space, reached for ignition switch to shut down, motor coughed and stopped, would not restart, cranked funny.

Timing gear had no nylon left, chain had jumped, four valves bent.



This summer... Pulled heads from the '68 350/4bbl/10.25:1 in my '73 Delta.
Notice ALL of the piston crowns (valves were fine):






- Eric

Del70

thanks, sorry to see your engine like that. I am glad my water pump went, and it was good time to RR the timing chain. It had the factory set, and nylon was all gone. it is a good idea to check timing chains, look into the fuel pump opening and see if it has the factory nylon cam gear. Mine did..

cutlassefi

Hey Eric,
Sorry for your troubles but those aren't 10.25:1 pistons. The 10.25:1 piston is basically like the L2320F, no appreciable dish at all.

MDchanic

Mark, I measured the dimensions of the dish. I don't have the numbers in front of me, but it calculated to about 6cc each.
The low-compression pistons are close in depth, but a bigger circumference, so that whenever it was that the pistons kissed the valves, they wouldn't have if it'd been low compression.
Also, you can see the V-shaped notches, indicating the 10.25:1 pistons.




And there's no trouble - they valves and the pistons kissed at some point in the past 100,000 miles.
No harm done.

- Eric

cutlassefi

Eric,
Maybe this will help.

4.057x4.057x3.385x12.87=717cc swept volume
6cc dish + .025 in the hole (5.3cc) + .017 head gasket (3.6) + 68-70cc (average head cc) yields approx. 9.5:1.
Minus the 6cc and you get about 10.0:1.

We all know the actual compression ratios aren't as stated. This confirms the math.

MDchanic

Okay, I see what you're getting at - I was using the 10.25:1 to describe the nominal values, ie: to describe the model of engine in the car, but you're talking about the actual calculated compression ratio.

Yes, absolutely, the actual CR is less than the claimed CR.

In my own case, I measured the CC volume and calculated the CR of each cylinder, and they're roughly 9.5:1 to 10.2:1 (I don't know where I put the #&@$ papers I wrote it all down on...).

The pistons in my engine are the original Oldsmobile pistons that GM lists as 10.25:1, as opposed to the pistons that GM lists as 9:1, which had a larger-diameter dish that would not be likely to hit the valves, which was my original point, which I fear has been completely lost.

- Eric

MDchanic

On my engine? Naaah. It was just a little tap - not enough to bend the valves at all, it just dinged the crowns by a few thousandths.

My engine was all original inside when I opened it up, nylon timing teeth were mostly off, but no evidence it had jumped a tooth - Odds are the valves hit the pistons while floating when some dumb@ss overrevved it, possibly decades ago.

It's not unusual to find, I just thought it was a nice way to illustrate the fact that the valves can hit the pistons.

- Eric

MDchanic

The oil pump screen was full of nylon (and more!), but still flowing fine.

I threw in a HV pump that I had on the shelf, anyway.

- Eric

Fun71

The 6cc dish pistons in the '68-'70 350 4bbl 310hp "10.25:1" rated engines are .077" deep x 2.54" diameter.

The "low compression" '68-'70 350-2bbl engines and the later '73-'76 engines used 14cc dish pistons that had .130" deep x 2.88" diameter dishes.

The dreaded "dog dish" '71-'72 (and most aftermarket cast replacement) pistons had .215" deep x 2.92" diameter dishes.


Just in case anyone was wondering.

EightballZ

talking about pistons hit the valves...

the engine of my buddies '67 pontiac exec. made a strange noise so we pulled the intake and found one bent and broken push rod on #6 exhaust valve...and several slightly bent push rods. we also pulled the fuel pump and found a very lose timing chain...cam lobes may be worn too

strange thing is...after we pulled the head on drivers side we couldnt find any evidence that the valves touched the pistons. gonna pull the head with the broken push rod next weekend..

sooo...if the push rod broke because the timing chain skipped a tooth ..wouldnt it be the same for every other push rod then?

engine ran till we teared it down

TripDeuces

Eightballz I'm wondering if the valve guides seized considering it's the exhaust valve. It's more common on exhaust than on intakes guides.

EightballZ

if that is the reason why it broke, the other push rods may be bent because their valves start seizing up too?

i was watching the valve movement on passenger side head while my buddy was cranking the engine (w/o spark)...valves were moving up and down without any problems.

TripDeuces

Eightballz, typically the exhaust valve has more clearance in the guide because it expands more from the increased heat.