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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 634
Registered: 4-2015
Posted on Tuesday, 28 November, 2017 - 06:33 am:   Edit PostDelete PostView Post/Check IP

Re: 1936 25/30 GHL3 Park Ward Saloon

Similar to the prior thread regarding a Derby Bentley, I am now working on a 25/30 and query as to the proper OEM specified spark plugs.
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christopher carnley
Unregistered guest
Posted From: 5.80.51.66
Posted on Tuesday, 28 November, 2017 - 06:57 pm:   Edit PostDelete PostView Post/Check IP

KLG type FLB30X

Champion LB8

(KLG was Kenelm L Guiness).

(Message approved by david_gore)
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 635
Registered: 4-2015
Posted on Wednesday, 29 November, 2017 - 05:33 am:   Edit PostDelete PostView Post/Check IP

Christopher...
Thanks for the information. Here is why I ask. Please correct any errors or misimpressions and fill in gaps in my theories...

I understand that in spark plug theory it is desirable to have the length of the threaded portion of the plug (reach) match the threaded portion of the cylinder head such that the end of the plug is flush with the interior edge of the combustion chamber. The reason for this (I have read and which seems logical) is that if the plug extends into the combustion chamber (and excluding the issue of possible contact with other interior components...valves, pistons) the exposed threads of the plug are susceptible to erosion and damage from exposure to the combustion gasses and in such a case the affected threads can cause hard removal and possibly gall the cylinder head threads upon plug removal. If this is true, I suppose that the alternate situation where the plug reach is shorter than the thickness and threaded portion of the head is likewise undesirable as this exposes the head threads to combustion erosion.

To summarize, I presume that it is desirable for the reach of the plug to match the thickness of the head and be neither too long nor too short.

Based on the premise this is what I have observed. The 3.5 Derby Bentley has a 1/2" thick head (the threaded depth is 1/2") whereas the RR 25/30 has a 3/4" threaded depth head. Accordingly I would presume that plugs with 1/2" reach should be used in the Bentley and 3/4" reach plugs in the 25/30.

The Champion LB8 has only 1/2" reach, therefore is appropriate in the Bentley but not so in the 25/30 which based on the aforesaid theory should use a 3/4" reach plug. Correct??

What I found is that my Bentley has 3/4" reach plugs (the Atlas 492 noted in prior thread) and the RR 25/30 has the 1/2" reach LB-8.

To make a long story short, I presume that it would be appropriate to switch the plugs and match the 1/2" reach LB-8 with the Bentley and the 3/4" reach Atlas 492 with the 25/30. Am I seeing this correctly??

P.S. In addition...it would seem that the OEM recommendation for 1/2" reach LB-8 in the RR with 3/4" threaded head thickness flies in the face of the prior theory.

Thanks for responsive opinions!

.
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christopher carnley
Unregistered guest
Posted From: 5.80.18.176
Posted on Wednesday, 29 November, 2017 - 06:59 pm:   Edit PostDelete PostView Post/Check IP

Christian,
If the OE plugs are 1/2" thread length, what make you believe that the thread length in the C.I head is 3/4"?
C.

(Message approved by david_gore)
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1650
Registered: 9-2004
Posted on Wednesday, 29 November, 2017 - 10:12 pm:   Edit PostDelete PostView Post/Check IP

"the exposed threads of the plug are susceptible to erosion and damage from exposure to the combustion gasses and in such a case the affected threads can cause hard removal and possibly gall the cylinder head threads upon plug removal. If this is true"

Yes true, also correct plug thread length is important for plug cooling.
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 636
Registered: 4-2015
Posted on Thursday, 30 November, 2017 - 04:31 am:   Edit PostDelete PostView Post/Check IP

Determining the threaded depth (thickness) of the cylinder head was a simple matter of removing a spark plug and observation combined with a measuring tool.

.
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David Gore
Moderator
Username: david_gore

Post Number: 2749
Registered: 4-2003
Posted on Thursday, 30 November, 2017 - 08:14 am:   Edit PostDelete PostView Post/Check IP

Further to the discussion above, Christopher Carnley has forwarded the extract below from the 25/30 Owners Manual:

Manual

.
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 637
Registered: 4-2015
Posted on Thursday, 30 November, 2017 - 12:21 pm:   Edit PostDelete PostView Post/Check IP

Yes. Thank you. I coincidentally found that exact page in the forum pre-war technical area last night while looking for data on the Phantom II that is next up to be recommissioned. This page verifies the OEM specifications to be either the LB-8 or the KLG 30X both of which have a 1/2" reach.

My specific question was however, given that the head appears to have a 3/4" threaded thickness, whether it is preferable to have a plug with a matching 3/4" reach.

At issue is not whether a 1/2" reach plug will work, but whether a 3/4" reach plug would be a better match to the thickness of the head, or whether the possibility of combustion erosion to the exposed 1/4" of threaded head depth is not relevant in the specific application for whatever reason.

.
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christopher carnley
Unregistered guest
Posted From: 86.164.119.102
Posted on Thursday, 30 November, 2017 - 07:43 pm:   Edit PostDelete PostView Post/Check IP

If you look inside the cylinder head sans plugs, you will notice that one side of the drilling /tapping is longer than 1/2" due to its corner location. R-R 1/2" plugs in a 1/2" head.
The reason that 3/4" plugs have been used is due to the differences in quality of the skill or knowledge of the owners/fitters availability, or ignorance during the past 80 years.
To quote that well known film, "The Witches of Eastwick"....... "But hey, if it fits, its in"!

The plug and head threads are close tolerance, and cases of hard carbon build up on the extra 1/4" of 3/4" plugs have caused plugs to jam solid, or to damage the threads on removal.

NGK plugs in the long body(for socket access) 6 range, with 1/4" thick copper or brass spacer washers will suffice.

(Message approved by david_gore)

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