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Radiator caps

Radiator Caps

The radiator cap on my 150 (835140) has a perished rubber seal and needs
replacing.I am however not sure if the existing one is the correct cap. It
is an AC 4lb F5 ( the rubber seal is appox 1 5/8" in diameter and the depth
from the underside of the cap to the base of the rubber seal is approx 1
1/8" closed and 15/16" open ).It doesn't appear that the amount it opens is
enough to clear the overflow pipe. Is this the correct cap or should I be
using something else? - Roger Herrick

Hi Roger Herrick & all -- this came back with my info (at top) clipped, so
here goes again -- this is from Dick Cavicke's post of 11/30, plus my own
experience: The correct 4 lb cap is available at Napa Auto Parts in the US,
Napa/Balkamp part # 703-1419, as well as Stant number TR101, 10281 (truck
radiator cap) -- cheap at $6 - $7.  Have used with great success on my
original Mk IX with no modifications; it just holds!  I expect I (or
someone) can get one or more & snailpost it to anyone not in the US.  I
hope this helps - Larry Martz

Blake: Here's an article that I sent to the Classic Jaguar Newsletter a
couple of years ago. It may be "more than you wanted to know" about a
new radiator cap but I hope you, and others, find the information


There is currently a good quality, authentic looking replacement radiator
cap available for XK120 and early XK140 cars. Several Jaguar parts
suppliers have it and the cap is also available from Napa Auto Parts under
the NAPA/Balkamp Part Number 703-1419, as well as under the Stant Number TR
101, 10281 (Truck Radiator Cap). Be certain to see that it is a 4 lb. cap
since the adjacent part number is a for a 7 lb. cap whose higher pressure
can cause radiator tank and/or heater core damage. The retail price in
Southern CA is less than $6. So much for the good news. In spite of its
proper appearance and low cost, there is a slight problem with the cap
because the lower portion cannot or will not always seat properly even when
you have given it your full attention. Evidence of the cap failing to seat
can come in the form of a frequent loss of coolant either out the overflow
tube or from around the cap itself during warm-to-hot running. The problem
is caused by the slightly larger-than-original diameter of part of the new
radiator cap and the effective diameter of the radiator filler-pipe neck.
The XK120 and early XK140 radiator filler necks were made with a number of
nubs protruding into them, possibly to impede the cap from being blown off
while removing them under pressure. Unfortunately, these nubs may interfere
with or totally obstruct the seating of the new cap. After experiencing
several unpleasant instances of coolant pushing past the cap when I thought
it was properly seated, I made a simple modification to the cap that
relieves the interference problem and greatly facilitates a good fit.
Here's how to do it: (If you need an illustration, I can send you a jpg
direct.) 1. Remove the lower cup-shaped portion of the cap (holding the
rubber seal and vacuum valve) by straightening the three tabs that secure
it to the spring flange. 2. Using a Dremel-type tool and a cut-off wheel,
remove the three vertical (flanged) portions of the cup between the
attaching tabs. Leave about one-eighth inch of vertical metal on each side
of the securing tabs. 3. Dress the cut edges and remove any burrs up to the
edge of the rubber seal. Be careful not to cut or weaken the tabs, damage
the sealing rubber, or distort the flat portion of the cup that backs the
seal. 4. Reassemble the cap, again being careful not to damage the rubber
seal when re-bending the tabs. With this change, you will find that the
lower part of the cap will clear the nubs with only minor jiggling. It will
seat and make a proper seal as you turn it down tight. - Good luck,  Dick

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