TheSamba.com :: View topic - help fitting spark plug head temp sensor--no room?

Wed Apr 02, 2008 10:09 am

I bought a VDO head temp gauge for my 2.0L '75, there is no way I can fit the sensor under the spark plug, no mater how careful I bend it!! Help! The counter bore around the spark plug hole is just too small.

Wed Apr 02, 2008 10:31 am

Germansupplyscott did a good write-up on installing these stupid things. I think it can be found on his website and here on the samba.
If memory serves, I think I had to make the hole a little bigger so the spark plug would fit through it. This also helps to keep it from binding when you thread the plug back into the head.
You can bend the sender with two pairs of pliers or a vice and whatever else you want to use.
When you go to thread the spark plug into the head, pull up on the sender's wire so it stays at the top of the plug's threads, where the washer was - I think most people cut the washer off the plug. If it's just sitting on the head when you thread the plug back in, it can catch a thread and actually get screwed into the head with the plug. The plug will not go in all the way but you might not realize it because it's so hard to see down there. You'll be left with a bad seal between the plug and the head, the sender won't work properly because it's all munged up in the hole, and the threads in the head are likely to get screwed up in the process also.
Sometimes I wonder if it's even worth having the gauge. I always dread pulling that plug out because it's such a pain to put back in.

Wed Apr 02, 2008 10:36 am

It'll fit, it's just tight. I just put a VDO sender on a new 2.0l. Make sure you bend it so the criped connection to the wires faces the plug, otherwise you risk getting that crimp caught on the head when you put the plug in and making a tear drop out of the ring. Then it won't seal correctly.

Wed Apr 02, 2008 10:39 am

You better stop cross posting the same question or the admins will slap your peepee.

I assume the engine is still in the bus. I think you're going to have to notch out the cylinder head to provide a channel for the tail of the ring terminal to sit in. Is that the problem or is the O.D. of the ring terminal to big to actually fit in the hole? Check out the tech article at www.germansupply.com

Also "use the search function" and find the thread in this forum that is titled "CHT Nirvana"

Report back

Wed Apr 02, 2008 12:36 pm

Thanks guys !! The plug fits thru the sender OK, but the crimp is too thick. Making a notch or grove for it is the only way i can see it working, but I hate to do that. Looks like a NGK plug will help also. That link has a lot of good info...Sorry about the double post.

Wed Apr 02, 2008 2:55 pm

Think of trying this sender instead.

http://www.sonexaircraft.com/eshop/cart.php?target=product&product_id=16597&category_id=336

Wed Apr 02, 2008 3:36 pm

nikita wrote: Think of trying this sender instead.

http://www.sonexaircraft.com/eshop/cart.php?target=product&product_id=16597&category_id=336

very cool!!

Thu Apr 03, 2008 6:28 am

So, with this type of sender, where on the head are you supposed to install it with the self tapping screw to get the best CHT reading?

Thu Apr 03, 2008 7:10 am

Jody '71 wrote: So, with this type of sender, where on the head are you supposed to install it with the self tapping screw to get the best CHT reading?

I believe that the recommendation by engine builders and those that have done data acquisition on our engines is to place the thermocouple under the sparkplug to get "the best (most accurate) CHT reading". Then again if you're using a guage that isn't cold junction compensated like the VDO, you could probably place it just about anywhere. A non-CJC guage is affected by ambient temperature. You have to add (or subtract?) the ambient temps to/from the gauge reading to get a more accurate idea of your CHT. I hate doing math while driving. :(

Thu Apr 03, 2008 7:56 am

Would that be ambient temp inside the car, at the gauge? If so, it still makes a difference where you mount the probe. I think the next hotest mounting point would be a head nut. I believe the VDO is calibrated for 75 F ambient at the gauge. so it its 90 deg under your dashboard, you would add 15 deg to the gauge reading. Right?

Thu Apr 03, 2008 8:22 am

It's ambient at the end of the thermocouple, and in your example you would subtract 15 degrees, not add.
In the case of the Westach, I think it's 4' long so it could be in the passenger compartment, albeit at the back.
If you keep your passenger compartment at room temperature, the reading will be fairly close.
Jim

Thu Apr 03, 2008 8:59 am

But my question still remains, where is this sender mounted to the head with a self tapping sheet metal screw? I am currently using a DD CHT set-up with the ring under the plug. It functions flawlessly, but that's because the ring fits the plug as should be, and it is not ovaled out. Anything other than that will result in questionable readings and higher head temps. Does this sender wire loom come in lengths suitable for Bus applications? Is it functional with a DD CHT gauge?

Thu Apr 03, 2008 9:52 am

I just modified a VDO sender with a new smaller ring and screwed it to the head beside the spark plug hole last month (shoulda took pics), engines still out of the bus so no comparisons yet but it's definitely going to eliminate the plug washer hassle.
I drilled and tapped a shallow hole on the vertical area just below/beside the plug, it's 3/8" away from the plug so I doubt temps are going to be much different.

Thu Apr 03, 2008 10:32 am

Jody,

Here's the picture from the link above:

Thu Apr 03, 2008 5:42 pm

I just attached mine on an exhaust stud using a washer between the probe and the nut. It looks good. I wonder if that area is hotter or cooler then at the plug? Anybody see any problems with this??

Thu Apr 03, 2008 5:46 pm

Not good enough to tell you what you want to know (the cylinder head temperature).

You need it under the plug to give you good information.

Thu Apr 03, 2008 5:49 pm

Randy, Not good enough in what way??

Thu Apr 03, 2008 5:57 pm

Well you are looking to keep your CHTs from going to the meltdown point of about 400º when the gauge sending unit is properly installed.

When I got my bus it had a CHT installed at the TSII location and it was not even close to what it was under the #3 plug.

It is up to you, your bus your money. If it were me, I would intall it where it is supposed to be or not even put it on.

Just my 2 cents though.

Thu Apr 03, 2008 6:22 pm

It seems to me that locating the sensor on the #3 plug is ideal in reference to the 400 degree melting point; however, what is really important is the delta of heat increase. In other words, if the difference between the normal reading at the #3 cylinder and the danger point is 50 degrees F, then you can assume that if you locate the sensor in a different location, you should get worried when the CHT climbs in a relative manner. If you get 275 at the TempII location and it climbs to 325, then you have a problem. The increase in temperature is what is important, not the absolute number. You would also want to take into account that the temps may not rise in the same proportion, but a drastic increase will alert you to a problem. Certainly better than no CHT at all.

Thu Apr 03, 2008 6:22 pm

how much, if any, temp differential would be caused by the heat from the sparkplug firing itself?

i would guess the plugs generate some heat on their own?

dp