Can not get pilot to lite was working fine had to turn off gas will I changed out some water pipe went to relite and could not get pilot to lite

Air in the gas. Bleed the pipe before lighting.

the explosion was from a build up of gas that was not properly ignited by the pilot light or was leaking into the combustion box when there was no pilot light
there is a serious problem in the gas control system that needs urgent professional assistance to fix
you were lucky as it could have blown you out of the room
get in an experienced gas fitter as it is not a diy job

The flashing light 4 times indicates high water temperature has activated the over heat sensor, and that the only remedy is to replace the gas control valve/thermostat.

It depends on many factors, including brand, type of heater, age, gas pressure, maintenance, venting, proper installation of thermocouple, bad thermocouple, air intake cleaning, if the flame is clear blue etc.
Copy following link to read more:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#pilot

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The paid answerers might have a specialist in your model of water heater.
As a free answerer, I specialize in providing resources that might help you for free.
When you say the gas control pieces, I assume you mean you replaced your entire gas valve with a generic dial-type gas valve.
You replaced thermocouple, but you did not replace burner assembly that includes the pilot tube.
Your other gas appliances are working fine.
You have not installed a new gas furnace with a blower that is vented up the same chimney as the water heater.
You have put your hand near the vent hood to see if there is backflow coming into the water heater when the furnace turns on.
You have removed the vent hood to give you access to the vent pipe, and you lit a match and the smoke from that match drafted upwards. You have done the lighted-match test when the furnace turns on which reveals the general condition of the venting stack.
You have checked your outdoor vent stack to make sure there is no obstruction, or the vent hood is not missing.
The burner flame on the water heater is a clear blue color with very little yellow. This shows the quality of the combustion air and reveals if amount of gas flowing to burner is restricted.

http://www.thetankatwaterheaterrescue.com/forums/forum3/

http://www.reliancewaterheaters.com/prod/gas.html
Reliance is an AO Smith company
The site doesn't have a manual for the 501
I e-mailed Reliance several weeks ago asking for a 501 manual.
They send a generic gas water heater manual with no troubleshooting

http://www.reliancewaterheaters.com/lit/om/g616.html
Link above shows a typical Reliant heater, the 616
Age of your water heater can be seen on serial number.
The two numbers following first letter are the year water heater was made.

http://www.reliancewaterheaters.com/lit/om/manuals/186486-002.pdf
Troubleshooting begins on pg 29
Troubleshoot flowchart begins on page 31
The new gas water heaters have the FVIR system so troubleshooting includes that information.
I don't think the 501 would have a FVIR flammable vapor prevention because it is older
The new systems have an igniter
The troubleshoot flowchart gives valuable information

after pilot lite you got to hold it down for 45 sec before you can turn it on..if you still having trouble get a qualify technician to relite it.

The thermocouple is not functioning. The t/c is the small (sensor) tube that the flame touches and should have a small copper tube (capillary) extending to the gas valve. (IMPORTANT: The capillary tube should not have any kinks or crimped places...Only gentle bends. A crimped tube will require t/c replacement.) The flame will deposit a coating (minerals) that will insulate the t/c and prevent it from working. Clean the sensor with steel wool, fine sand paper (emery board) or scotch pad. Now try to relight the pilot making sure the t/c sensor is contacted by the flame, if not, shut off gas and adjust. Once aligned and contacted by pilot flame, hold for a minute, allowing the t/c to warm up fully and signal the gas valve to operate when knob is turned from pilot to operate.

hi there i come from the uk, but it sounds to me as though it maynot be a thermocouple as if it was the pilot would light when you held it in & go out when you let it go.
it seems to me as though the pilot injector could be blocked no gas getting through to it ? or possibly there is a break in the insulation of the igniter so that when you try & light it it just earths out to the heaters metalwork.

These are just a few ideas that might shed some light on your problem?

Kindly

Richie