Spark Plugs

Gary from Iowa · Feb 13, 2017

Is it a normal rule that the lower the number on a series of spark plugs, i.e., D-21, D-16, D-14, D-9; the cooler the plug? I have a DC-3 that has 4 1/16 dome pistons and some other items. Put it on a Dino after getting it all back together and run it for a little over a hour. It is tighter than I like, really have to pull on the crank. Converted to 12 volts and will start with starter but it labors. Runs good, but I am afraid it might seize if I pulled it hard. Was told by a old mechanic, two years younger then me, to run colder plugs in it until it freed up a little...Make sense or just an old tale. Oh, piston clearance, mains, rod clearance and end gap on rings was all checked during assembly for proper spec. Thanks.

oldproudvet · Feb 14, 2017

The first thing I'd suspect is the line bore of the block, brg clearance could be fine, but babbit mains have to be properly line bored, usually on something like a Tobin-Arp piece of eqpt. Shimming and scraping works on individual brgs, but not the whole. How did the crank spin, with mains torqued w/o the rods? Con rod misalignment is also not uncommon, were the rods jig tested for twist and straightness? Of course piston skirt clearance is important, but usually not an issue with sleeve/piston assemblies. When tore down, you may be able to find the issue, a polished skirt indicating alignment, but you have to find the issue. If you're going to pull it, it will sieze...... Cold plugs/warm plugs will have nothing to do with break in. Plugs are matched to combustion temps, high compressions, harder working etc require a bit cooler plug, then a parade unit or light duty usage.

John Saeli · Feb 14, 2017

Gary: I agree totally w/ ProudVet on the Line Bore, and Plugs. The only plug I recommend in your tractor is Autolite 3116. Also, make sure you have real copper core wires, not the silicone or radio suppression wires. Timing & poor ignition will give the laboring symptom you mention. As for the tight engine, ProudVet most likely nailed it.

tomturkey · Feb 14, 2017

Oldproudvet, I've never rebuilt an engine, It is challenge I might like to attempt. Would have plasti-guage shown a possible problem on the crank shaft alignment? How easy should the crank roll over once the mains are all torqued down, then after the rods and pistons are installed? As I said I've never rebuilt an engine, with the price of parts I may never. Just accumulating knowledge from your experience to decide if rebuilding one will go on my bucket list. By the way, I too am an old proud vet. gobble

casecollectorsc · Feb 14, 2017

Maybe need to remove the hand covers and use a mirror to see if it starting to score the sleeves.

Gary from Iowa · Feb 14, 2017

Thanks for all your comments. Engine line bore and other items were conducted by a machine shop, or they said they did. This is a shop that has been in business and considered top notch for many years. Yes, crank was removed, rings etc. we're installed and reassembled. Before the head was put on you could turn it over, but it did take some effort. This tractor was not built for pulling, but to surprise some red and green tractors at local plow days. John, it has 3116 plugs in it, but I did not know the colder plug numbers for autolite. Hey, I am still learning and taking advice. I think casecollectersc and oldproudvet have provided good advice. tomturkey, I am a old proud vet also.

Don Rudolph · Feb 14, 2017

We recently had to put new sleeve O-rings in a DC3 with M and W's and put all the original shims back in the connecting rods. 3 of them had 3 shims and 1 had 4 shims. Just for fun we tried 3 shims on the rod that had 4. That tightened the motor up to the point of turning really,really hard. I couldn't believe 1 shim could make such a difference, but it sure did. Don

Spark Plugs

Gary from Iowa

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oldproudvet

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John Saeli

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tomturkey

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casecollectorsc

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Gary from Iowa

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Don Rudolph

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