B18/20 Heater Core Return Pipe 
6-2023 R Kwas  Revisions On-Going [Comments Added]

Recently, some owners/installers have reported installation difficulties or even leaks at the Water Pump connection of the Heater Core Return Pipe.  Here are some details of this installation considered. 

Firstly, the Sealing Ring used should be the rectangular cross-section type, (included with replacement WaPus), and it should lightly lubed/sealed with some gasket sealer, before installing onto HCRP, seated until it contacts the Circumferential Ring of the Pipe.   During installation into WaPu, the Seal outer surface or pocket in WaPu housing should be lightly lubed/sealed as well, before seating it home.  The Gasket sealer will help lube it as it is inserted into its pocket in the WaPu, seating it home, compressing the Sealing Ring snugly...and the installer will know and confirm it is home because the hole in the securing tab will line up to allow the HCRP to be securely held in place by the WaPu.  In the opinion of the writer, the beauty of this design is that it is quite simple and (I would say) difficult if not impossible to get wrong! 

Soft Pipe to Clamp Interface


Two pictures accompanying a question of repeated leaks:    

Steven S. pictures (permission requested).

Pic 1. Movement in the indicated direction of 0.015" is possible (but shouldn't be!) and measured. 


Pic 2.  Some fore and aft movement is also possible (but shouldn't be!) as evidenced by chaffing of the paint from the pipe. 

My response to a question accompanied by pictures above: 

"Movement you are measuring in the left pic [1] should not occur as the tab coming off the front of pipe allows it to be [movementless!]secured it to the WaPu bolt, and pipe is effectively soft-mounted in rubber sealing ring whose compliance allows for thermal expansion.

The movement (abraded away paint!) shown on right picture [2] should also NOT be possible if pipe is securely located at the front, but to prevent abrasion, I recommend a "soft interface" between Clamp and pipe by wrapping a piece of inner tube."


Reference pictures of a separated assembly: 

Heater Core Return Pipe to Water Pump interface.  Ultra-simple, capable of withstanding Cooling System pressure while allowing thermal dimensional changes...truly a brilliant design in its simplicity and effectiveness!  The square cross-section seal (NOT the oval sealing ring), is installed up to the Circumferential Ring on the Pipe and so seals the Pipe, and as Pipe is inserted into its pocket, when the hole in Pipe Securing Tab lines up with WaPu mounting hole, all is right...it can be seen from the highlighted D dimension, that when the tab mounting hole lines up with the WaPu housing hole, the Sealing Ring is properly positioned in the pocket. 


Closeup of rear pipe Clamp on pipe showing there is plenty of space between Clamp and pipe for my suggested "Soft Pipe to Clamp Interface" improvement.  

Locating Clamp near rear on HCRP.  This Clamp is secured to the rearmost Manifold Stud.  From the picture, it can be seen that Clamp is snug dimensionally on the pipe even before its two ends are tightly clamped together when mounting.  This results in a snug, non-chaffing mechanical connection...but I have seen some pipe wear due to chaffing at this mount (due to a loose or even neglected Clamp)...as a minor upgrade, I now wrap a strategically sized and cut piece of sheet neoprene around the pipe to assure a soft and compliant interface, so that what is shown in the picture cannot happen.


I have observed significant wear on more than one of the pipes, at the rear securing Clamp location due to vibration/chaffing.  This makes the point for assuring the pipe is securely held by the Clamp, and without slop, which would eventually result in a compromised pipe!  

Heater Core Return Pipe exhibiting significant wear obviously due to a loose rear Clamp which allowed chaffing [I just wonder what the remaining wall thickness is there!].  Tip:  When assembling, assure pipe is held securely and there is NO movement possible!


Interface at the rear of the (5/8" OD) HCRP is by way of a simple rubber hose with a hoseclamp, which runs off to the Heater Control Valve, so this should present little difficulty. 

See also:  https://www.sw-em.com/Cooling_System.htm#heating_/_defrost_systems 


Soft Pipe to Clamp Interface:

Details, Details!

...and since there is plenty of material available in a retired inner tube, [ Free SW-EM Shop Tip No. 32!]  I also have found that two wide bands harvested from the same Inner tube work well to make a quickly deployed soft vice jaw cover also visible...



This article is Copyright © 2023.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of its products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 

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