I'm breaking in a new engine rebuild, and when the spark is advanced beyond the best position, the engine makes a dull thumping or knock.

As long as the spark is not advanced too far, it sounds fine. The engine was "tight", and I had to pull start it the first several times but can hand crank it now.

Thanks for your help in advance (yes, pun intended) .

Rob

"when the spark is advanced beyond the best position.."

Well, like the Dr. says, Rob, "Don't do that."

Which car/engine is that?

rdr

It doesn't need that much advance and its telling you. Too much advance makes the cylinder fire before TDC and the result is knocking from the mains and rods.

Rob,

Ut-oh...this is getting scary...Ralphie and I have now agreed on about 5 in a row

I too wouldn't find myself too concerned if a fresh rebuild did that. I've had one where it started fine on the 2nd notch on mag, but you didn't want to bring it much past 8 o'clock or it got a low drone at idle.

Just learned to drive it that way, it never overheated, always had power when needed, and while I've thought to bend the rod a bit to give me more travel of the stick for less travel of the timer, it was just easier to remember that 8 o'clock was the 'sweet spot'

Thanks guys. It just seems more of a solid knock than our other Ts.

Ralph, this is our "new" 09, #2507. Just trying to get the "kinks" out.

Rob

Rob,

Another thought. It could be just a timing issue and that way over center spark...but if you are concerned that it may be more...and since it's the 09...short the spark out one at a time while its set to 'thump' and check. If only one makes the thump go away, it's worth another look at the lower end on that one...but then its the joy of an '09 getting to it!

You could also try determining if 90 degrees up top = 90 degrees down below on the timer, that sounds a bit simpler than going in to check the lower end.

As expensive as open valve engines are i would want to find out why instead of waiting?? The Wifes 29 A has the problem untill warm for over 10 years but compaired to a early T the A engine is dirt cheap! I run the A spark somewhat retarted but with no overheating is it really retarted?? Bud.

Bob Heyen: Your statement about the fact that the knocking occurs when you advance beyond "best position" on a fresh rebuild simply tells me that the rebuilder attached and set the timer in a slightly different position than it was in before the rebuild.

You now have a new "best position" It's that simple. Positioning the timer on the front of the cam is an arbitrary happening and all you have to do to make it the same as it was would be to bend the spark advance-retard arm slightly so that it goes back to where you are used to it being. Or simply leave it alone and learn its new operating procedures.

If you can not advance it as far as you could before it means that you are starting it in a slightly more advanced position and perhaps should bend the rod so as to retard the timer position relative to the spark lever, then it will be just as it was and as you are used to it being.

As I always say, Henry hid top dead center very well and it gives all of us a head ache from time to time.

Rob - Just a guess, but I'll bet that as the new and tight engine loosens up, the "thump" you're hearing will slowly become the old familiar "knock" like your other "T"s,......harold

Look under the hood for the thumping noise, perhaps it's the rebuilder trying to get out!

Thanks guys. George, that is a good idea. I'll try shorting one cylinder at a time and see if one is causing the noise (sure hope not).

Thanks again, and have a good Easter weekend,

Rob

the only model t ive seen do that is a real nasty dump truck, and i assume the bearings were less than adequate.

my t just starts missing when i over advance it, but it doesnt knock.

Rob
I have a similiar problem on my 25 Tudor. I has a rebuild engine with a few miles on it now. When shorting out spark plugs while the knock is occuring, it goes away when i short #1 cyl. This winter i tightened the rod bearings hoping for the best. Still had the same noise totally unchanged. When i mentioned the problem to an old friend with Model T experience he asked me if it went away when i retarded the spark. I replied yes, and he immediatedly said "Piston Slap, I've had that before". A while back there was something on the forum about this and someone said that some of the alumnium pistons were made in China and were bad for this. I have a feeling that this is the problem because the sound is exactly like a piston slap. I will probably pull it down next winter, I'm enjoying driving it too much now.
I realize that all problems are not the same, but maybe this gives you some more "food for thought".
I hope this helps,
Roger

Roger,

I recall hearing (or reading about) piston slap before. Is it caused by pistons touching the head, loose wrist pins, or ???

Thanks,

Rob

i think piston slap is where the piston is not quite a perfect fit in the cly' and it moves sideway a little

piston slap occurs when you have too much clearance between bore and piston, it will problably wear your bore and pistons faster, but other than that its just annoying.

yep, definetly dont use pistons from china. i havent had to buy new T piostons yet, but i think Jahns is still around and they used to make great pistons. i know Silvolite makes the best cast model a pistons, and they may be able to make you T pistons.

i havent heard many good things about Egge.

if you cant find a good source, i believe Y-block pistons fit, and there available in different grades

Rob, double check your timing. The commutator rod adjustment is different for the brass '09-'10 timers. The guages available are for the later model Ford timers. Set yours by piston travel and
you will be dead-on. Be prepared for "free starts".
Lee

Rob
I tool the old gal out for a good run today (maybe 20 miles) and had an enjoyable time. The noise is most prevalent when not under a load or holding back, just running free. I can be running along on level ground and hit a slight grade and the noise quietens noticeably. It sounds exactly like the piston is rocking around in the cylinder. Reducing spark timing takes it away or almost. Also, I do believe that the knock might get slightly better when the motor is good and warm. This would be a strong indication of piston slap.I don't think that it bothers me enough to spoil my driving time now by pulling it down. I'll save that fun for later.