I have just removed the spark plugs of my 3.5. It were Bosch Platinum WR7DP
I also had a set of spare plugs W8DC?? which I installed for now with 0.035" gap (0.88mm).

Are these WR7DP and W8DC correct plugs? What plugs are recommended for this engine?

I have some problems with cylinder number 5 thread... not sure the plug in cylinder 5 screws in hard and the thread looks bad after removal... I don't have the tap for the cylinder head thread. Any advise. I'd hate to remove the head.
Martin

The ideal is non resistor plugs. Bosch has stopped selling non resistor plugs but you can still buy them from NGK. I use BP6ES or BP7ES on my Euro cars (Not BPR which are resistor plugs). I also gap to 0.40 as my M117's seem to like it better.

You should be able to buy a thread chaser (even in Oman) like the one pictured.

__________________
With best regards

Al

I use BP5ES plugs. I find that BP 6 or 7 ES plugs are too cold for almost every application.

Hi Al, this is great info.
I'll chase the chaser here.
I am sure I can get the NGK plugs somewhere here.... it will take a while.
Is 0.04" (= 1.01mm) a good gap? I head that idle is better with wider gap?
Any opinion on BP6ES or BP7ES beeing too 'cold'? Using BP5ES?

Gap for resistor and non resistor plugs:

I found this information on the forum...
'Hello all, I was looking in the Haynes manual and they specify a bigger gap for resistor plugs..They have the gap for non resistor plugs at 0.28-0.32 and the gap for resistor plugs at 0.35-0.39...Am I wrong in assuming that the increased gap would create more resistance..A layman would look at that and think it would be the other way around...There seems to be a lot of controversy with the resistor plugs'

Any thoughts?
Martin

old threads
1) (Arthur Dalton) "Yeah, another way of putting it...'Inverse' works too, and Bosch are the opposite..higher/hotter.

These ignitions were not High Energy by a long shot, so I never go beyond .030 on any of them..and the spark heat is higher at a lower gap, but the kernal time is longer at a larger gap..so, a better idle is gotten with a larger gap...with .030 being close to ideal, with a Non resistor plug. "
2) (Mike D.) " i.e., NGK BP5ES are "hotter" than BP6ES. It works the same way with the newer NGK stocking numbers. A 7333 is hotter than a 7433 etc.

Widening the gap reaches a point of diminishing returns. If you set them at .035" then after a few hundred thousands of firing the gap will widen to .036" due to wear and metal transfer from the spark.

.030" is a good setting which will give you pretty close to a combination of peak firing AND longevity."



...so the opinion is that 0.03" gap for the NGK plugs is ideal?

And which of both NGK BP5ES (hotter) or BP6ES (colder) is good for the 3.5 engine?

Martin

Martin

I did not put too much thought into it, I just discussed it with an MB mechanic friend some time ago when I bought my first 450SLC 5.0. He indicated that on the US cars, with the lower compression motors, he used hotter plugs (BP5ES), but not on the euro cars.

The US 5.0 engines have a 8.0:1 compression ratio and Euro cars have 8.8:1 compression ratio. I believe that the 3.5 had a 9.5:1 compression ratio.

Setting the gap to .4 was another tip that I took up. No problems.

__________________
With best regards

Al

Here are some picture of the Bosch Platinum WR7DP I was running after renowal of the heads. What do people say about this?

Attached Thumbnails

   

They look OK from the picture, just old. But take a look at this as it describes how to read plugs. As they are in front of you, you should be able to get a better diagnostic.

Spark Plug Analysis

__________________
With best regards

Al

Thanks Al