I got a 360 i'm rebuilding tje bottom end on. Ive always used a machine shop before but trying on my own this time

The main and rod bearings all have 020mstamped on them, as does the end of the crank. What does this mean? Has it been line bored 020 oversize?

All the journals on the crank measure out to stock (not ground). The rods are all original d7te except for 1, which tells me someone has been in here before, likely due to a spun bearing in the past. The cylinders all measure out to indicate its been bored out 020 there also, but my pistons all have 040 stamped on them-but the crank turned freely with tem in with engine on the stand.

The original problem was that when warmed up, it would start missing then seizing up. Im thinking oversize pistons bound up when warm and caused seizing?

 

The 020 on the bearings mean they are for crankshaft journals that have been ground .020" undersized from their nominal diameter. Being that you measured your crank journals to be at nominal diameter this is a big issue.

If the bores are .020" oversize and the pistons are .040" oversize, I don't think they would fit into the bores. I don't know what the piston to cylinder clearance is for a 360 but .020"+ seems way too large.

I wouldn't hold much credit on the stampings, I would measure everything and reference the factory specs and tolerances. What are you using to measure everything?

 

Measure, measure, measure. With the quality of parts out there today, don't do anything until you measure!

Back in the good ol' days (whenever that was), you could drop off a crank at the local machine shop, the guys would say "she'll clean up at ten!" and you could stop at the parts house after picking up the crank and get some .010 bearings and everything would be ok.

Not any more. I always measure the crank to see how much cut it will need to clean up, then order bearings, then install and mic bearings, then cut the crank to the measured desired bearing clearance.

Same deal with pistons. Mic the piston, bore and hone to exact desired fit. I always file fit my rings also.

Sadly, the first set of bearings and the first set of pistons may be junk. Remember, you need one complete set of GOOD parts to build one good engine. At the factory, if something was out of whack, they had a whole range of parts to choose from. Not so for the home builder, especially now days when replacement parts are of dubious quality.

Just remember two things, don't trust anybody and parts can't lie. They will tell you their story if you listen!

 

Thanks. Im using an old stainless us made micrometer.

Im pretty sure the crank matched the specs in the shop manual, that really threw me off. The cylinders measured at around 4.08/9.


UOTE=MixWizard;14872431]The 020 on the bearings mean they are for crankshaft journals that have been ground .020" undersized from their nominal diameter. Being that you measured your crank journals to be at nominal diameter this is a big issue.

If the bores are .020" oversize and the pistons are .040" oversize, I don't think they would fit into the bores. I don't know what the piston to cylinder clearance is for a 360 but .020"+ seems way too large.

I wouldn't hold much credit on the stampings, I would measure everything and reference the factory specs and tolerances. What are you using to measure everything?[/QUOTE]

 

I use certified starrett id/od mics and snap gauges where necessary. I have access to these via my work. You can usually find mics on ebay/craigslist pretty cheap. Take them to your machine shop and ask if you can verify against their standards if you don't get a set that comes with standards.

Starret (Athol, Mass.) is the best, Mitutoyo is good too. That's what the rest of the world seems to use. Your machinist should be able to assist you in calibrating them if they are off. If not, just note how far they are off and correct your measurements.

It is best to have someone who is familiar to show you how to use them properly, so you get the feel right. For example, know I mic a 1/2 thou tight usually. I don't measure stuff on a daily basis, and rarely use the same mics twice. There is kinda an art to it.

Your piston to bore clearance will vary by bore size and type of piston (cast, hypereutectic, forged) but .005" is on the large end for big block sized bores. Some will go to .007 or so with nitrous, etc. but for a regular 4" bore .0015-.002 is probably ballpark. .003" will probably have some audible piston slap.

You should also check your cylinders for out of round and taper. The easiest way to do that is with a dial bore gauge, but multiple measurements with snap gauges and a good outside mic will do to (just takes time).

 

Thanks. My mic is probably 40+ years old, my dad usef it when he was an a&p mechanic, never hsd it calibrated.

When you say snap gauges, you mean those t gauges ?