Rad cap

[Ill Named]

So I finally finished up the beast. For the last bit (brakes, exhaust, alignment) I took it to a shop, mostly because I was sick of working on it. When I picked it up, the shop owner told me that the top rad hose blew off of the thing when it was just sitting there after they had finished running it. I thought that odd, but then I thought that i must have not tightened it properly.
I drove it home and was looking it over in my driveway after I shut it off and boom, off came the top rad hose along with all the lovely hot coolant all over my face. I never thought that I would actually be happy that I was wearing glasses, but now I am.
What could be causing this? I am leaning toward the rad cap being to heavy and not allowing it to vent. Sound reasonable?
Specs:
400 sbc .030 over, long rod, alu heads(drilled steam holes), air-gap intake, high flow oil pump, new cam, roller rockers, true double roller timing chain, Mallory hei, new rad, water pump, hoses, t-stat, rad cap(16 pounder), alternator, hooker headers, flowmasters, fully balanced rotating assembly.
I am sure that there is more there that I can't remember right now. It is also odd that it did not do this when I was high idling it to break in the cam. It doesn't run hot or anything like that, it stays at a constant 190.
Any input would be great.
Thanks
Ill 

[Lt.Del]

I am leaning toward the rad cap being to heavy and not allowing it to vent

I think you hit it right on the head.  Your engine gets very hot when shut off, not water pump running, no fan running.  Lots of pressure built up.  That's why you have a coolant resevoir. The spring in the cap will open up and allow coolant to the resevoir.

[VileZambonie]

Get a radiator cap with a built in gauge. If it's going over the rated pressure of the cap you are leaking compression into the cooling system.

[Lt.Del]

I didn't know they made caps with guages.  Did a ebay search and...

...here's a nice radiator cap pressure guage...



http://cgi.ebay.com/ebaymotors/VINTAGE-BOYCE-MOTO-METER-AUTOMOBILE-RADIATOR-CAP-GUAGE_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1234Q7c39Q3a1Q7c66Q3a2Q7c65Q3a12Q7c240Q3a1318QQ_trksidZp3286Q2ec0Q2em14QQhashZitem230295264725QQitemZ230295264725

.....if you have a Model T 

Seriously though, you can get some "radiator pressure tester"s on ebay.  I didn't see any caps with guages, I guess there is a speacial place you can get them.

http://motors.shop.ebay.com/items/_W0QQ_fromZR46?_nkw=radiator+pressure+tester&_nd1=&_fromfsb=&_trksid=m270.l1313&_odkw=radiator+pressure&_osacat=0

[VileZambonie]

You can buy caps with temp gauges and pressure gauges built in. If you can't find one just get a pressure tester, put it on and run it.

[DnStClr]

Ill, did you have your block cleaned out thoroughly before it went back together? I didn't read your other posts about the rebuild. But it sounds like there's a blockage in your water passages  doesn't it? Or else the coolant mix has too much water. Just throwing around a coupla thoughts.

[Ill Named]

Yes the block was thoroughly cleaned, cooked, bored, and decked. I replaced the 16# cap with a 13# and all seems well now. Driving it around yesterday, I found out the hard way that my fuel tank selector switch does not work, if it ain't one thing it's another eh. I am also getting a lot of oil coming out of both of my valve cover breathers, maybe I do need a pcv valve after all, what do ya think?

[Captkaos]

Are you valve covers baffled?  I not, then YES you most definitely need a PCV otherwise you will start blowing seals out.  It is NOT an emissions item, it is Positive Crankcase Ventilation.  You have to have it to vent the pressure from the crankcase.

[Ill Named]

Gotcha, I will get the valve tomorrow. Thanks for letting me know, I just need the one on one valve cover right?

[Captkaos]

Yes, you only need one...
Are you breathers elevated above the valve covers?

[Ill Named]

yes they are on the top of the covers which are tall polished aluminum.

[Captkaos]

If they are just flush with the valve covers this could cause it as they need a riser.  If they are aftermarket aluminum I am guessing they are not baffled.

[Ill Named]

Now I am fully perplexed. I solved the pcv valve problem today, but discovered that I still have the rad hose problem. On my way to the dump, which is quite a long steep hill to climb, my top rad hose blew off again. Only this time it was while I was driving it. What the heck could this be? Faulty thermostat? On my rad where the hose clamps on, it has a step down to accommodate a smaller size of hose, which I don't use, and where I clamp my larger hose there is only about 3/4" of a bite for the clamp as opposed to a nice long neck for it to clamp onto. Could this be the problem? Also, I remember that when I placed my thermostat into the intake and then put the housing over it, I could reach in through the neck of the housing and move the thermostat up and down a good 1/8". That shouldn't cause this problem though should it?

[VileZambonie]

If the hose is not tight it will pop off. Try a different hose or use a hose reducer adapter. Carquest, Napa etc carries them.

[roadrage41]

ill are you running one of the chrome water necks, and is it the correct one? mine has a small cutout that keeps the area of conact at the thermostat from being completely round - it holds the thermostat firmly to the manifold when its bolted down. i think the earlier ones dont have that.