Hi All, When I was finding TDC (with piston stop) on my SBC 350 in my C10 truck I noticed a lot of air coming out of the #1 spark plug hole (with all other spark plugs left in place) on the compression stroke, however on the exhaust stroke I still felt a little air coming out as well (though no where near as much). Is this typical for an original motor with high miles? I'm guessing likely cam wear causing the valves to not open correctly at the right time? The engine runs ok, I can vaguely recall an intermittent vibration at idle that's not too bad and I don't think its an issue with springs, etc, I recently finished doing valve seals and set valve lash correctly. I'll at least check with my infrared thermometer if it appears all cylinders have somewhat similar temps. On my SBC 350 in my 50 shoebox ford I only get air coming out of #1 cylinder (with all other spark plugs left in place) on the compression stroke and nothing on the exhaust stroke but that motor was rebuilt (though not sure when). I'm more curious than anything else if I should expect this behavior for an original high mileage engine (I suspect high mileage), I have no plans to diagnose this deeper as this truck sees about 100 miles per year, it idles ok and drives ok right now, and eventually I would consider putting another motor in it or having it rebuilt or just getting rid of the truck altogether.
Relax, there's always some stroke pressure. The piston is 4", the valve is 1½". I'll hazard a guess you were bumping over vs turning by hand? If I was from Southy I'd say, "fuggetaboutit"...
Ah ok. Just interesting on the SBC 350 in my 50 shoebox I didn't feel anything out of the #1 hole on the exhaust stroke.. I would have thought most of the air should be going into the exhaust manifold on the exhaust stroke and I shouldn't feel air coming out of the cylinder hole but I suppose air would flow out from both. Yes turning by hand.
Hah no. Turning by hand! Here is/was my method : Turning by hand clockwise with left hand, feeling for air pressure with finger on right hand of #1 spark plug (with it out). It was easy to tell which is the compression stroke vs exhaust stroke even though both had some air pressure coming out of the cylinder spark plug hole. After feeling the air pressure starting to come out and prior to getting to the general region of the TDC mark on the compression stroke I would put the piston stop tool I made in and turn until it stops and place the mark.. then turn the opposite way and place the other mark and then between the two get my true TDC.
You’ve never witnessed wind coming thru more than just one window in your home when several of them are opened?
It's normal and nothing to worry about. You are holding your finger over the hole to feel the compression? You will feel a little pressure against your finger on the exhaust stroke but not enough to write home about unless the exhaust system is plugged up.
Oh for sure, and that's why I asked because everything I read online that tells you how to check for TDC says to put your finger over the spark plug hole and feel for air coming out.. well I could see how someone could interpret some air coming out on the exhaust stroke to mean you must be coming up on TDC on compression and you can set your distributor correctly now. I've never really heard anyone mention you should make sure you should check two revolutions and be sure you're feeling air on the compression stroke which noticeably should be the stronger of the two!. I made that mistake a while ago and was off 180 with my rotor because I felt some air coming out on the exhaust stroke. It's strange my SBC 350 on my Ford shoebox doesn't do that though, only on the compression stroke do I feel any air at all out of the #1 spark plug hole.
Thanks! Yep I did feel a little pressure during the exhaust stroke.. enough of a burst that the first time I was doing this a few years ago I thought that was where I needed to be and set up my distributor and ended up being 180 off. I haven't read too much info on-line that suggests you should check multiple revolutions and make sure you feel a stronger burst of air and not just some air. Maybe that applies more to an original engine with high miles like mine. The SBC 350 in my shoebox was rebuilt and doesn't have any air pressure over the #1 spark plug hole at all on the exhaust stroke.
I'm sitting here just thinking that over the past 62 years I have brought up an untold number of engines on compression to set the timing, taught somewhere north of 650 students how to do it including class contests of seeing how fast teams of two could put a 289 up on compression, drop the distributor in, set it to where it would fire, lock it down and put the plug wires back on and fire it. The record being under 2 minutes. With that in those 62 years I have never once cranked an engine over by hand with my finger over the plug hole that I can remember. I've cranked a lot over by hand while watching the rocker arms on # 1 cylinder and that works quite well. With that you feel air against your finger because the piston is moving far slower when you hand crank it than when you crank it with the starter and are only checking for actual pressure.
That makes sense! In my case I have to do these things solo.. not to many friends I call on to help with any car projects.. I suppose I could get a remote switch so I can crank myself instead of having to do it by hand next time around.
I watch the valves. Both #1cylinder intake and exhaust valves are closed on the power stroke, to mark TDC or drop distributor in correctly. The exhaust valve is just closing on the exhaust stroke. No help needed.
We used to have a contest like that in HS auto shop . we had a 300 ford 6 on a stand . I won it my senior year . I cheated a bit and took a 50 /50 guess on it being on TDC compression and new were the paint chip on the pully had to be and were the rotor had to be already so . so I spun it to my spot and dropped the distributor and hooked up the wires with out ever checking if it was 180 of or not . Was like 58 seconds or something . ( lot faster han a V8) I could do it almost as fast if I had to crank it over with a finger in the the hole. If it came up on the first one or two starter bumps. Lot of fun and we learned lot in that class had a awesome teacher . I have cranked them by hand and been able to do it but its a PITA . I used to do my race boat that way but nothing was in the way of me spinning it with a breaker bar nearly 180 degrees at once .
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