Hi everyone,
does anyone know the size, thread type / pitch of the block drain plug C31616 above the dipstick tube ? I'm going to flush my system before replacing all the hoses and would like to get a fitting ahead of time so I can attach a drain hose and keep my new exhaust wrap etc clean.

Cheers,
Brian

 

Never measured mine.

Waaaaaay back then, I found that the garden hoes end bit, what screws into the lawn sprinkle, threaded in just fine, then I plugged the garden hose onto it, and left it flushing while beer was consumed.

Push and shove, I would say 1/2" NF.

 

Brian,
if I recall correctly, it is 22mm.

 

Dont know about 22mm,

All mine, from the MK X onward had a tap in there, and it was the SAME tap as the radiator drain, and that was not 22mm threads for sure.

 

yes my 1965 3.8 S type has a tap or "faucet" there, and my 1984 4.2 XJ-6 has a bolt.

I never had a problem removing that bolt. and I am almost sure I used a 22mm socket with a 1/2” ratchet and a hammer to smack it loose. Lot of gunk came out thru the hole. I used Prestone Super Flush twice to clean the block.

 

Jose, How exactly did you flush the block. I'm installing a new Alum Radiator and am thinking I should clean my block before I do.

 

Iramphal, it is a long process but here it goes: (with new or old radiator but radiator installed)

with engine cold, remove expansion tank Cap to release any pressure.

remove thermostat housing and thermostat, careful with the studs. Have a new thermostat and new gasket ready if you don't know the history of the old one.

replace thermostat housing, without thermostat or gasket, doesn't matter if it leaks, it's temporary in situ.

drain remaining coolant by whatever means you prefer. I removed the lower radiator hose at the radiawtor and water pump, and the bolt on the side of the block. Once drained. replace hose and block bolt, no need to tighten much.

fill expansion tank with a bottle of Prestone Super Flush.
​​​​​
insert a garden hose in expansion tank and fill system with water until tank fills.

start engine and turn Heater Temp to maximum and fan speed to High.

start engine and run for around 20 minutes to circulate Super flush cleaner until you get heat inside the cabin. Keep garden hose handy so you can fill system as water and cleaner fills the heater system and maintain radiator full if any leaks.

Turn engine off and allow to cool so cleaning solution does its job inside block and radiator etc. No matter if there are leaks.

when engine has cooled, about 45 minutes, remove block drain bolt and lower radiator hose again, let everything drain, it will be a mix of remaining coolant, water, Super flush, and gunk.

Once nothing else comes out, close the system again, replace block bolt and lower hose. and repeat with another bottle ​​​​​of Super flush.

I do it twice to make sure all components and parts surfaces are cleaned as best as possible, especially the heater core and heater valve.

once the above is repetaed, Drain again a second time.

refill system again a third time. but only with water to dilute / remove any Super Flush detergent.

Drain again after running engine 15 minutes. Drain system good.

Then tighten block bolt good,
tightn lower hose clamps at rad and pump,
install new thermostat and gasket,

fill with the mix of coolant and water you prefer. I use 60% coolant to 40% water.
Or 6 quarts of coolant and 4 quarts of water mixed.

Once full, run engine with Heater ON to max.

System needs to settle with new thermostat and coolant level will fall. Keep adding coolant. Let system burp air bubbles. keep adding coolant. Let engine reach Normal temp so thermostat opens and lets coolant through. Put expansion tank Cap on but do not tighten fully. Take it for a drive, Come back and check coolant level, check for any leaks in the parts that were removed.

Keep checking coolant level again without pressurizing system, to burp air out. Heater ON all the time you are doing this. Very important.

Last thing to do is to tighten expansion tank Cap and take it for another drive.

Next morning, check coolant level again with engine cold. You might notice coolant level went down. refill it.

If i forgot any detail, well you know, it happens.

 

Many thanks Jose. This will be my first project come Spring!

 

I have this planned for spring as well. I'm pretty sure that the drain plug on the block hasn't moved in 35 years on mine so might be a bit of an effort to get it out. Given the large capacity of the XJ6 cooling system I'm thinking of adding two bottles of cleaner at a time instead of one just to make sure there is good strength to clean things up.

Barcho

 

the Prestone Super Flush has given me good results in my cars with only 1 bottle at a time, but I do it twice so it's the same thing.

There is another procedure called a Reverse Flush and Prestone makes a kit to do it.

I have done it in my Ford van but not in the Jag, since you need to sacrifice the UPPER RADIATOR HOSE to temporarily install a fitting which reverses the water flow, so instead of moving water from the radiator to the engine and back, water is moved through the top hose to the radiator, out the lower radiator and back up thru the water pump, removing the opposite gunk that normally remains in the system.