The Stovebolt Site Forums The Shop Area Engine Shop More info on bad rebuild

OK, more info:
180 degree thermostat, checkd in a water bath with an accurate thermometer, amnd it's working OK. New water pump. The original engine was prone to overheat, but that's fixed. Cam is new, original equipment specs, and degrees out OK. I'm suspecting a hot spot in the cylinder head, causing the #1 cylinder to run hot/lean. The scoring is on the thrust face of the piston, and corresponding cylinder damage. On both previous teardowns, the wrist pin is tight, but nor siezed in the piston. Rods have been resized, and checked for straightness and twist. Everything Ok so far! We're about to pull the head, and have it checked for restrictions and/or cracks in the #1 cylinder area. The head is the only thing that's common to all jobs so far. Pistons, crank, rods, etc. have been checked and/or replaced or reconditioned on every assembly, with no problems noted. Mixture checks out OK on a 4-gas exhaust analyzer, so overall mixture is OK.
Jerry

HrL, You are correct in that the first engine failure sounds like it was due primarily to the piston having insufficient clearance. There are many reasons for scuffing and scoring, I will try to list a few that I have encountered over the years.

Scuffing and scoring are caused by

1. The lubrication system

a. worn oil pump
b. clogged oil screen or passage
c. oil pressure relief vavle stuck open
d. incorrect bearing clearance
e. contaminated oil
f. low oil level

The cooling system

a. external leaks
b. internal leaks
c. clogged radiator
d. deposits in block (this is one I think may be where your problem is at)
e. defective thermostat
f. defective radiator cap
g. worn or broken fan belt or corroded water pump impeller

3. carburation

a. too rich, to lean, or stuck automatic choke

4. Incorrect ignition timing

a. wrong ignition advancement
b. worn distributor altering spark advance

5. Combustion knock, detonation or pre ignition

6. Improperly fitted parts

a. insufficient piston clearance in the cylinder (this was the one I thought for the first tear down)
b. tight piston pin fit (or this one also)
c. rings with insufficient gap
d.improper cylinder liner fit

7. Improper break-in

a. slow idle
b. coolant not being allowed to reach operating temperature

8. Lugging or overloading

There are probably many other reasons that I haven't encountered but this may help others also. Use a welding rod or coat hanger and see if you have heavy deposits around the number one cylinder. Since it is "out of sight" it is easy to overlook. Good luck and let me know what you find out.

Also, one other thing I thought of- did you take all the burrs out of the combustion area of the head? These can lead to preignition and over heat the cylinder.

Thanks, Oly!
Sounds like you've twisted a wrench or two in your time. I did a thorough check of the #1 cylinder water jacket area when we switched to the different block, and found no problems. The rest of the junkyard block was in pretty good shape, but #1 was worn so badly it needed a sleeve. Is this commom to 250's? All the rest of the cylinders cleaned up at .030 oversize with no problem.

I've used the 307 V-8 pistons on several 250 buildups, with good results. One in particular in a Nova could run with the V-8's after a little bump in cam profile and compression. It belonged to one of my students in the trade school where I was teaching who wanted a hotrod a little out of the ordinary.

At least this time, there's a sleeve in the problem cylinder, so I can re-sleeve it without going to yet another block. Just a lot of work to do another complete teardown! I'm leaning toward replacing the head, even if my head shop guy doesn't find a problem with it. It's the only common factor in all three failures. I'll probably go with a different rod in that hole, too. All the other cylinders are showing NO problems at all.

I've got four Onan turbo diesels, inline 6-cylinders UPS used in their big brown vans! They used them interchangeably with 292 GMC's, and **** 300's, all with the same engine mount and bellhousing setups. They used the old late-50's Chevy hydraulic clutch bellhousings, 12-inch clutches, and New Process 235 4-speed transmissions! Direct swap into a Chevy pickup! That would solve the problem, bigtime!
Jerry

Hotrod,
I don't want to get off-topic here, but what is involved with using the 307 pistons in a 250? I'm (slowly) building a hi-po 250, and it sounds like a good swap. What kind of compression ratio do you get from the flat-top pistons?

Depending on the mauufacturer, the 307's are total flattops, or sometimes they have a chamfer around the outside edge of the piston. They've got the classic V-8 valve reliefs in the top, but they don't have the dished center like the 250's have. I've found they raise the compression about 1/2 to 3/4 of a ratio, more if the head is milled at the same time. They're a direct swap, same weight, bore size, compression height, and wrist pin diameter. Just toss the 250 pistons, and install the 307's.
Jerry

That sounds good. I had heard over on the Inliners page that some machining of the piston skirts might be necessary, but if you've just dropped them in with no problems, that takes another item off my to-do list. Thanks!

Hotrod Lincoln,

Off subject but, I have to enquire about your Onan diesel engines. I have a 42, 11/2 ton military chevy (see my website) which I am replacing a 235 with a 292. I've looked at all of the diesel options, either too costly or an extreme conversion. The Onan sounds like an ideal candidate, I have never seen used UPS trucks advertised for sale or in boneyards. My question, where do you find a used UPS drivetrain?

Thanks, Ernie L.

All the UPS vehicles are custom-spec trucks, and I doubt if they get recycled through conventional methods. These engines were a one-time purchase by a friend, and I got them through some creative horsetrading. The Onan company was acquired by Cummins some time ago, and the engine, which was originally designed as an electric powerplant power unit, was sold to UPS as a truck engine. I think there was a problem with Cummins building enough of them to supply the UPS demand, and the trucks were retro-fitted to a different engine. Mine are rated at 120 HP at 3,600 RPM, and I've dyno-tested one to confirm it. It's an inline 6-cylinder with a small turbocharger. It's about the size of a 292, but somewhat heavier, so a front spring upgrade would be necessary, I believe. I don't know where to tell you to look for one, but I'd start with the local UPS maintenence shop, and see if they can give you some information on availability, if any. The engines I have are set up for Chevy bellhousings and 12-inch Chevy 1-ton clutches, so a swap into a Chevy truck should be pretty simple. I'm going to install one in an 87 F-350 flatbed I've got, which has already worn out two 351 Windsor engines! it's already got a GM 14-bolt corporate rear axle in it, so with a Cummins diesel and a New Process transmission, it ought to have a real identity crisis! I'd love to find a Clark 5-speed to use, with overdrive 5th. gear, but they're a little scarce and expensive, especially with the Chevy bellhousing bolt pattern! I've got a full machine shop and welding facilities, so I can fabricate just about any adapters I need. Between custom gunsmithing and race engine building, I don't know when I've got time to work for a living! I teach auto mechanics in a big high school in Nashville, Tennessee!
Jerry

Hotrod,
Do you have a preferred brand for those 307 pistons? I don't want to use junk in my rebuild.

I've been using the Speed-Pro Hypereutectic pistons, with good luck, even on the mid-horsepower race engines. On the really high-dollar engines, I use forged pistons, but for a hot street engine or a "claimer" on the track, the hypereutectics work well. I modified the dome shape on a set I put in a 305 recently, and had to use a carbide lathe bit to cut them. That high-silicone aluminum actually dulled a high-speed steel tool bit! Now, that's some tough aluminum!
Jerry