Comparison DMCH and Specialt hot water valves

[DrJeff]

I had the opportunity today to do a side-by-side comparison of the DMCH and SpecialtAuto hot water valves. Another owner had suffered a failure on his Specialt valve and advised that I get the DMCH one. Neither of the valves shown have been fitted into a car, so they are just as they came from the vendor.

DMCH (left) and Specialt (right)
DMCH and SPecialt SideBySide.jpg

Here I compare the wall thickness (at the failure point). I made sure to measure away from the bulge to get the true thickness. The Specialt valve on the right has been dissected at the time the photo was taken (more on that in a moment). The wall thickness is the same in both cases (2mm).
DMCH wall thickness.jpgSpecialt wall thickness.jpg

This photo shows the Specialt valve as it comes installed in the hose kit. You can just see some foam like adhesive between the top of the hose and where it attaches to the valve (just where the arrow is conveniently pointing).
Hervey Valve Installed.jpg

Once I pulled the hose off you can see the extent of the 'adhesive like stuff'. Since I wasn't going to use the Specialt valve and I was curious about the adhesive, I decided to see what lay within! I scrapped away the adhesive foam - which was actually very soft and could be removed with just my thumb nail. There was no evidence that the glue was there for any reason that I could see. I cut into the valve stem to see if the stem and the valve body had been glued together. However, as far as I could tell the stem and the body were cast from the same plastic. So it begs the question... why? Was the adhesive an attempt to strengthen the valve at a known failure point?
Hervey Adhesive Around Outlet.jpgSpecialt Adhesive Off.jpg

I was surprised how soft the plastic was at the base of the stem, of course I don't have anything to compare it with.

[ALEXAKOS]

Kudos for the review!!!!
I have heard some complains about DMCH pars..bur I rarely head good things about Special's repro's...

[Bitsyncmaster]

I've seen many warnings when using plastic parts to use no sealers or glue because it can attack the plastic.

[DCUK Martin]

Look familiar?

[djdogbone]

I'd be willing to bet that the source of failure IS the adhesitve as it is probably weakening the plastic over time.

[Bitsyncmaster]

I just installed a sump pump under my house. It said in the instructions not to use any sealer on the plastic threaded fitting (1 1/4" pipe) because it can damage the plastic. It's made of that same black plastic I see on the heater shutoff.

[NightFlyer]

Personally, I wouldn't use either valve as a replacement, unless you insist on such for concours purposes.

The stock/OEM, DMCH, Hervey cross reference, and Cadillac cross reference are all butterfly valves consisting of a separate vacuum chamber and valve body connected by a mechanical linkage. While there's nothing wrong with this design per se, the pivot shaft for the butterfly valve runs directly through the valve body, thus creating a point of common leakage when the internal seals eventually wear out (my stock/OEM valve was recently leaking coolant from the pivot shaft prior to replacement). Also, the mechanical linkage can easily corrode/rust, get stuck, or even get hung up on other lines/hoses within the vicinity of the valve (it's quite busy back there).

That's why the DMCToday.com crowd prefers to use the AutoZone Everco (by 4 Seasons) #74612. It is a one piece self-contained vacuum actuated piston valve in a thermoplastic housing. No metal to corrode/rust, no mechanical linkage to fail or get hung up, no internal seals to fail, no pivot shaft to leak, and a much smaller footprint in an already crowded area of our engine bays. Not to mention that the AutoZone valve is only $10 and is usually in-stock for immediate pick-up at all AutoZone locations. Not a single reported failure of this valve by any of the satisfied owners running them, including myself.

http://www.autozone.com/autozone/par...ntifier=142760

[DMCMW Dave]

The stock/OEM, DMCH, Hervey cross reference, and Cadillac cross reference are all butterfly valves consisting of a separate vacuum chamber and valve body connected by a mechanical linkage.]

I have a problem with that design shown. Failure rate is not my issue, I'm sure it will last as long or longer than any of them. My problem is the failure mode. This valve (or any valve) can fail in several ways:

--crack or rust and leak
--get stuck open or closed (I've seen both)
--failure of the vacuum diaphragm.

It's the third mode that would worry me. (The other two faults could happen to any design to the same effects). In the event of diaphragm failures, the older designs with the external linkage simply quit closing the valve and will cause a small vacuum leak in "max" mode.

In this design, failure of the diaphragm (or other internal seal) will cause coolant to be pulled into the engine via the mode switch by vacuum (and force by coolant pressure. (If the mode switch was closed it would probably dump hot coolant inside the console). This would cause some very strange and extremely hard to identify problems, i.e. it would look an awful lot like a blown head gasket.

I can't say I've seen that happen, but sometimes thinking about the "what if" cases is important.

[Bitsyncmaster]

In this design, failure of the diaphragm (or other internal seal) will cause coolant to be pulled into the engine via the mode switch by vacuum (and force by coolant pressure. (If the mode switch was closed it would probably dump hot coolant inside the console). This would cause some very strange and extremely hard to identify problems, i.e. it would look an awful lot like a blown head gasket.

I can't say I've seen that happen, but sometimes thinking about the "what if" cases is important.

Never thought of that failure but it looks like it could happen. A peice of clear hose could show that failure.

[DMCMW Dave]

Never thought of that failure but it looks like it could happen. A peice of clear hose could show that failure.

So would coolant in your console - or exhaust.