Compression in a T

Keith True

Keith True

Well-known member
What kind of compression should I be looking for in a 23 T? I've always used the old Ford handbook to work on these with,but I've never found a compression value.I have one with 25 pounds across the board.Low? normal? Is it even important as long as it is even and has some? # one is a little off,not quite a skip,but if shorted to ground when running there is a change but not as much as when the others are shorted.All the coils were sent out and redone years ago,but not used until recently.Swapping them around doesn't change anything so that kind of eliminates them.I'm leaning toward a valve hanging up,too tight,or something in the seat so it leaks at idle.I checked the compression like I was shown as a kid.Take out a plug,start it up,and jam the tester in the hole.Comes up to 25 instantly.
 
Never heard of doing it that way! The normal way is to take all the plugs out, put the tester in the hole and crank it over 4 or 5 time for each cylinder. Check them dry then wet. 25 is very low. 35 to 40 is marginal but doable. 40 to 50 not bad for a use engine. Remember you are working with less then 5 Hp max per cylinder. Compression should be maybe 5 LBS or less difference between cylinders.
A miss could be any number of things; timer, coil, carbon track in coil box, plug, valve, bad connection or short in wires etc.
 
Try it as redmodelt says with the throttle wide open to give it as much air as possible. if it drives OK, I'd leave it alone. Another easy test I like is to hold your hand over the exhaust. If the engine slows or stalls the valves are seating. Only works if the exhaust is tight.
 
FWIW - I just checked compression on my '24 Touring car. All read between 52 and 57 psi.
 
This one won't kick the compression tester at cranking speed.The fellow I worked for in the early 70's used to work on a few T's.When he got one that wouldn't kick the guage at cranking speed he would test one at a time running.He said that if it had compression then at least he knew something was working.Then he would work backwards.The only things I know of that run on that kind of compression are some one lunger engines.I just didn't want a T guy telling me later that the thing will run fine,just a little weak.It does start up fine,and it is only used right now for driving in and out of display areas at trade shows.There are no abnormal noises in it.I did find slips from a guy that rebuilds coils in it,the air gaps measure good and all the coils throw a huge blue spark Swapping them around makes no difference,number one is still lazy.Not dead,just lazy.It has the redone coils,wiring,timer,plugs,coil box,ceramic tubes,etc.I know new doesn't mean good a lot of the time so I went through all that one piece at a time.Also,all the slips are dated 1993.It was never finished and it sat until last summer.I really was pretty well set on feeling it was a compression issue,just needed it confirmed from folks familiar with T's.I really think this engine was taken apart by the previous owner,cleaned up,and reassembled,collapsed rings and all.
 
Needs a valve job. Pull the head and if theres 2 pin holes on the valve head for a lapper, they are old cast iron head valves and are probably done for. Get new SS valves, cut some clean seats, and lap them in. ws

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I put oversized valves in and came to find that the guide was off center from the seat by about .010". Hence the deeper seat on the one. That .010 gave me a big fart ZERO on the test. Now they all run about 55-60 psi on the hand crank!
 
Do a proper comp. test before you pull anything apart. It'll tell you how far you need to go into the motor. Open the choke and throttle fully. Take compression on each cylinder. WRITE DOWN THE NUMBERS! Don't rely on memory. when you've done all 4 squirt some engine oil into #1 and check compression again. Repeat this on the remaining 3 cylinders. If the numbers go up with the oil it's usually worn rings because the oil will form a temporary seal on the cyl. walls & rings. If it doesn't go up it's usually valves leaking. The oil won't affect them. The eng. doesn't need to be hot when you do this test. If it is you will get slightly higher readings if it's worn rings and no difference if it's valves so hot or cold won't matter. In fact on a T the compression is so low normally that you want to know the cold engine pressures because that's the way it'll be when you start the car for the first time and low comp. will definitely affect starting. (Overall power too).
 
What are you using for a compression gauge? With the spark plugs out even hand cranking you should be able to get a reading with a hand held or screw in 200 LB gauge if it has compression cold. With the compression as stated by you sounds like a fight to get the engine started as it has to over come the drag of the transmission. In comparison single cylinder engines don't have a lot of drag and a heavy flywheel to compensate for the low compression.
 
Pardon the long nipple ! :eek: After the hone job with new pistons and rings, new OS lifters and new OS valves with new springs and keepers, I still had a dismal 5 lbs on #1 and about 10lbs on #2. The rig you see here is for a cylinder leak down test which proved a few bad valve seatings.
With the bare valve sitting in the block, it looked good. I took a small brass hammer and gave the valve a tap and I could feel it seat another .010". Id literally have to pop it back off the seat and then checked it with a feeler gage. I could get the .010 gage in around half the valve head. That proved that the previous rebuilder cut new seats off center. I am guessing this all happened in the '70s and when the poor results showed up he gave up. Thats why the Pollinator was in the shape it was when I got it.
The seat reamer I have merely followed the offset guide. As it turned out, it became a finesse job forcing the reamer on a tap wrench off to the high side and checking several times until the valve seated at 100 % then I relapped with 55+ lbs showing on all. ws

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The last pic shows the poor seat contact with a single hand twist of the reamer by finger. AH HA!!
 
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