I have an interesting problem , I have just rebuilt the bottom end of my 360/390 . I took a 390 crank and rods and a 360 block and pistons added all new bearings and ,cast iron rings , freshly honed the cylinder walls , a new timeing chain , high volume oil pump, a new distributer ,rebuilt carburator, rebuilt starter ,all new gaskets. It wont start . I have spark , fuel , and whats really got me stumpted is that the compression seem to come and go . I would like to add that its aprox 20 degrees outside . when I lose compression I have poured oil in the spark plug holes and it seems to help compression to the point that it almost starts. Im probably answering my own question here , is it possible that the oil is to cold to make it to the cylinder walls and help with compression? When I installed the pistons I was carefull to not only soak them in oil but to make sure the rings were gapped the right direction. Has anyone had anything simmilar happen ? Thanks for any advice , Bob

 

Oil around the rings should NOT be the deciding factor of compression. If it IS, then the rings are worn(or not seating). That is always the test for compression: if you add oil and it helps, then rings are worn. The oil soaking of the rings is for break-in. You do not have enough oil splash at start up to keep the rings/pistions from scuffing the walls. That is why you oil them.

You mentioned "gapped in the right direction". Elaborate a bit. Does that mean you lined them all up? or made sure they were all in different directions?

Did you happen to check ring GAP before installing them on the pistons? This would tell you if you had the correct rings for the application.

If you have fuel/spark/(air is given), then it should start right off.

You definitely have a problem, it's just determining it.

Larry

 

I made sur they were opposite to each other and that the correct side was up. It is starting to be my thinking that I ordered 20 thousanth over and I recieved and installed standard ones with out checking them

 

thanks for the reply, i kind of bounce off the walls up here sometimes, Bob

 

I cannot say I'm an expert on this, but if you say you put the rings opposite to each other, this sounds wrong. The instructions that came with my .030 over Moly's was to make a centerline, so to speak, across the top of the piston, then set your compression ring openings on that line. Set the oil ring openings on the opposite side of the piston, also on that line so they are 180 degrees from both your compression rings. Then you move the compression rings approx. 1 inch from that centerline in opposite directions, then the same with the thin lil oil rings that go above and below the wavy oil ring(man, do I have technical terms, or what?) This is the process I used on my 390, and it has worked well, besides almost blowing the engine running it too lean.

 

many thanks for the replys . its nice to have other opinions . helps me keep my sanity . I think I was trying to hurry late at nite in the cold and missed checking the ring box for the size. every thing has to be ordered in up here , which is why im disgusted with myself for not reading the box. this is going to make me appreciate those long summer days fishing that are just arround the corner , Thanks again,Bob

 

is it possible the timing gears arent lined up or your distributor way off???
it would seem to me even with worn rings or almost no rings even, that it would still atleast start.........or sputter for a minute......hmmmmmmmm
assuming you checked the compression stroke to make sure it's firing when it's supposed to???

 

Timing could be an issue. You might want to confirm this also. Make sure timing mark is at TDC, the piston is TDC and the dist. is pointing at #1. This will not confirm the camshaft, you need to pull off covers and confirm rocker orientation. You might have the valves opening/closing too far off to start? Its a thought.

Larry

 

ill go and check everything one more time before i pull the engine out. stranger things have happened , thanks again for the replys, bob

 

You didn't mention new pistons, so I'll assum they are reused? If the pistons ring grooves are worn enough then no new rings or proper gaps will get the compressions up to where it needs to be. The result is an engine that is about impossible to get started. The rings have got to get a good seal on the pistons as well as the cylinder wall or the problem will be just like you are experiencing. Hopefully you just have a timing problem or just some out of adjustment or sticking valves? Did you oil the valve stems and rocker arms before trying to start the engine. Are you positive you don't have some bent pushrods?

There is always a chance you did something wrong in the way you staggered the ring gaps or didn't have enough gap and some rings are broken. The proper ring gap placement is to first find the piston pin bore centerline and mark the pistons with a permenent marker if you want to be dead on the money for the gap spacing. The gaps for the oil rings are to face the rear of the engine and be one inch away from either side of the pin bore centerline with the oil ring spacer gap to the rear and centered on the pin bore centerline. The compression ring's gaps are spaced one inch away from either side of the pin bore centerline to the front of the engine. This essentially places a 2 inch space between the oil ring's gaps and then a 2 inch gap between the compression ring's gaps. You must follow the manufacturers instructions for ring orientation as far as which ring goes in which groove and which side of the ring faces up or down. Mess this up and you will be taking the engine apart again because the engine will burn a lot of oil at the least.

 

It is entirely possible that the timing is off enough to get your syptoms. When I put my 360 togeather this summer I set the timing with a ohm meter and when I tried to start the beast I couldn't keep it running long enough to check the timing with a timing light untill I got my wife to aim the light at the timing marks while I kept it going. It was about 15 deg. ATDC. When I moved it to 10 BTDC it ran like a champ. Why is that problems always happen when its COLD out? Never in the summer.

 

Did you say you used 360 pistons with a 390 crank and rods? Those rods are shorter for the longer stroke but I dont know about the pin placement in the piston itself. If it is different, the compression would have a hard time building up if at all. Just a thought.

 

If anything the 360 pistons will have a taller comp height than a 390 piston. But there are so many variations in piston comp heights , that it 's impossible to tell you for sure.

 

I would like to thank all of you for your replys and advice , it is now zero degrees out side and this forum is a real good reason for me to take a break and warm my hands . The pistons were used but the same as the 390 as far as pin height the only difference was a small divit on the face of each one, all straight push rods , well oiled , im at the point of pulling the timeing cover off to double check the timing marks. one thing i didnt do was line the rings in the manner described by RHunt64, many thanks for that info , ill do them that way . my way was to make sure they were oppisit each other useing the pin for a centerline, well im warm now ,so back to it . thanks again , bob

 

Just yhought I would warm my hands and do an update , of sorts. my timeing marks werte off slightly , but the biggest problem was that i ordered standard soze rings for a block that had been bored 40 over , the way i found that out is thst i ordered a dial caliper and measured the bore. it measured 4.090. Jost thought i would thank everyone again for the suggestions and advice . i also put a welded up a wood stove for my shop so that i wouldnt be in as big a hurry to get done and get warm . i would like to double check that the bore reading i got is 40 over ,. does any one knoe if that is what 4.090 means ? its been so long since i played with one of these things , im not sure. thanks again for the replies. bob