| Author | Message | ||
     Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 626 Registered: 4-2015 |
1935 Derby Bentley B44EF 3.5 liter engine First: What are the recommended spark plugs? Owner's Manual gives no data. Currently installed are "Atlas 492" which may or may not be proper and which are obsolete and do not show in any interchange data I have found. Second: How does one know that a spark plug is in fact defective? Other than obvious visual defects (chipped porcelain, cavitated electrodes) is there any way to test for proper function? Yes, they are an inexpensive component and simply replacing them periodically is standard procedure, but is there any way to determine "known defective" or "simply worn out"? . | ||
| Jim Walters Prolific User Username: jim_walters Post Number: 131 Registered: 1-2014 |
Champion LB 8 was the original spec plug. The best way to test used plugs is with a spark plug tester that fires the plugs while it is under 100+ psi air pressure. I still have an old AC plug testing machine that does this. Early in my career I when I might have 5 or 6 British cars in a day for tune ups I would show the customer why they needed new plugs with the testing machine. Very dramatic evidence. You could clearly see a bright spark at zero pressure then as you turned up the regulator the spark would disappear usually around 100 psi if the plug was faulty. You could actually hear the spark shorting out through the electrode insulation. I still have some NOS LB8's, contact me offline. SRH8505 SRC18015 SRE22493 NAC-05370 www.bristolmotors.com | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 1616 Registered: 9-2004 |
And this is my old Champion plug tester cleaner now found, last used to check a mower plug many moons ago!   | ||
| Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 628 Registered: 4-2015 |
Coming back to this spark plug issue, first, thank you Jim and Patrick for your input. Next, just what is the purpose of the proper spark plug gap and what is affected if the gap is incorrect, say, too narrow at .015 instead of .025? Will the intensity of the spark change? Will the timing of the spark change? I can appreciate that the proper gap setting matters, but why? . | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 1624 Registered: 9-2004 |
From my old memory with the older cars the 25thou is the one to go for. Gives a hotter spark for the ignition of the air fuel compression mixture. In most cases a 15thou gap is for the magneto type systems. The effect of a closer gap will be a weaker spark and not a clean burn. Another interesting thing with plugs etc is the polarity of the system. + earth and the plug spark travels from the side electrode to the centre electrode, this causes the side electrode to burn away faster. - earth and it is reversed with a better spark. Many years ago with the 1080sun engine diagnostic tester hooked up to a car running a magneto I have in the back of head that half the plugs were running + earth effect on 3plugs and the other 3 - earth but may be wrong on that! | ||
| richard george yeaman Grand Master Username: richyrich Post Number: 862 Registered: 4-2012 |
Patrick I think I was lucky I didn't get any of that cider of yours, (only joking). Richard. | ||
| Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 629 Registered: 4-2015 |
Patrick... Thanks for the info. I suspected that it would be something along the lines of improper combustion from weak spark. My questions have come from the effort to activate the Derby Bentley from a 20+ year slumber. When acquired about 30 years ago it had the Atlas 492 spark plugs, ran fine, and was used frequently over several years for dates, special occasions, and general fun usage before I lost interest in it. When recommissioning it, I consulted the owner's manual for plug gap where it is indicated to set them to .015 to .018 which I did without giving it another thought. Later I managed to leave the ignition on for a few days which precipitated the question about possible distributor points damage asked in an other thread. I then thought that it might be time to renew the spark plugs which precipitated the interest in the Atlas 492 plugs. Apparently their history has fallen into some sort of black hole. All I knew for fact was that they were in the engine when acquired and seemed to perform properly. What was unclear was whether they were the proper plug for the Derby Bentley. Jim has identified the originally specified plugs as being the Champion LB-8 which are obsolete. Finally it dawned on me that the information in the owner's manual was also "obsolete" as it presumably applied specifically to the LB-8 plugs and NOT necessarily to any replacement, substitute, or modern equivalent plugs and that the proper gap setting for any OTHER plugs should be as specified by their manufacturer, and NOT per the manual information. It then became probable that I had gapped the Atlas plugs too narrowly and that .025 is likely the better setting. This evening I did just that and the engine fires right up and idles like a dream. Whether the wider gap will address the fouling and hard starting issue remains to be seen. The lesson here, which has been noted in other threads about proper oil for the Bijur system and for the EPW transmission fluid issues is simply that the problem with reliance on manual data is that subsequent TSDs and upgrade recommendations are absent. In this case, relying on the manual plug gap setting was only valid as long as the recommended LB-8 plugs were being used. For any other plugs, the proper gap for those plugs as indicated by their manufacturer takes precedence over the "obsolete" manual data. Enough said! Thanks. | ||
| Mark Luft Prolific User Username: bentleyman1993 Post Number: 136 Registered: 10-2016 |
Christian, a quick search reveals both plugs are available on Ebay. | ||
| Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 630 Registered: 4-2015 |
Mark... Can you send me a listing number for the LB-8? I have done extensive search for all of LB8, LB 8, LB-8, with and without "Champion" or "Spark Plug" added to the search term and found nothing. If you did, please refer? As to the "Atlas 492", yes, I found 5 of the necessary 6 accumulated from 3 different sellers, but the issue here is that there is no way of knowing that the Atlas are a proper substitute for the LB-8. All that can be said about the Atlas 492, is that they were in the engine when acquired in 1990 and seem to work fine. Your further assistance appreciated regarding the LB-8. Thanks. . | ||
| Mark Luft Prolific User Username: bentleyman1993 Post Number: 137 Registered: 10-2016 |
Christian, I do apologize. When I went back, all the links to LB-8's were old and had been sold. I did check with Champion and they cross referenced their 896 spark plug as a replacement. http://www.fme-cat.com/overlays/part-detail.aspx?brandId=CP&pNum=896&partType=Spark%20Plug I hope this helps, and again, I'm sorry I jumped the gun. Mark | ||
| Jim Walters Prolific User Username: jim_walters Post Number: 134 Registered: 1-2014 |
https://www.ebay.com/itm/202116624387#shpCntId SRH8505 SRC18015 SRE22493 NAC-05370 www.bristolmotors.com | ||
| christopher carnley Unregistered guest Posted From: 86.147.219.175 |
All of the Derby engines have short, 1/2" plugs, but as few are now available, the long reach Champion N8 3/4" reach with 1/4" thick copper or brass collars have been in use for many years. There are a lot of N8 equivalents as NGK etc. Will Fiennes makes new cylinder heads, threaded for the longer reach plugs. (Message approved by david_gore) | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 1630 Registered: 9-2004 |
Richard Y, Wow three pints of Cider and was more convinced that the plugs with a magneto have a different spark direction!!!!!!!!!! Nothing with Sun but found this with Google. Spark Polarity. As a magneto rotates, the interaction between the lines of magnetic flux and the wire conductors in the coil is continually reversing. As a result, the voltages generated by the coil are alternatively positive and negative voltages. On twin cylinder magnetos, both these voltages are utilised thereby delivering a positive voltage to one spark plug, and a negative voltage to the other. This means that the spark jumps from the centre to the side electrode on one spark plug and in the opposite direction, from the side to the centre electrode, on the other. The design of a spark plug is such that it’s centre electrode (surrounded as it is by a ceramic thermal insulator) is always hotter than the side electrode (screwed directly into the cylinder head which acts as a heat sink). Now, the laws of physics show that electrons are more easily emitted from a hot electrode to a colder one so a spark jumping from the centre electrode to the side - caused by the negative voltage - is always stronger than the other. This picture of the sparks from a six cylinder magneto clearly shows the difference between the alternate sparks: http://www.themagnetoguys.co.uk/spark-polarity | ||
| richard george yeaman Grand Master Username: richyrich Post Number: 863 Registered: 4-2012 |
Patrick L OK you have convinced me, I will have another one please. Richard | ||
| Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 632 Registered: 4-2015 |
Jim and Mark.... Thanks for your input on the LB-8 as it has helped me "connect the dots". Christopher... Knowing that your specialties are the Pre-War and EPW models, I had hoped that you might reply. Thanks. Your information has likewise confirmed what I have been able to construct independently. First though, I am thus far happy that simply adjusting the plug gap setting has materially affected (hopefully resolved) the plug fouling, hard starting, and excessively smoky exhaust. With plugs gapped at .025 the baby fires immediately, purrs quietly, idles smoothly, and once warm and choke off, has almost smoke free exhaust. My interest has been to learn more about the Atlas 492 plugs and whether they are similar to the original specifications, or were simply a convenient replacement many years ago as noted in above posts. Based on the accumulated information kindly provided, I can now conclude that the Atlas 492s, while a workable substitute, are dimensionally dissimilar to the LB-8. The 492s have a 3/4" reach and a medium protrusion on the electrode. The LB-8 have a 1/2" reach and although none of the available photos clearly indicate, appear to have a short projection. Whether there is any detriment from the plug extending an additional 1/4" into the combustion chamber, there is no interference with the top of the piston, presumably due to the low compression of the engine design resulting in a sufficiently large combustion chamber to accommodate this extra length. The need for the 1/4" spacers as used by Fiennes may be necessary for other reasons but not interference. What are the issues affected by the extent that the plug extends into the chamber? If none, why would Fiennes use them other than to allow the use of 3/4" threaded plugs? My other question regards the protrusion of the electrode beyond the end of the plug body. On the 492s, the end of the porcelain (not the electrode) is about 1/8" beyond the end of the plug body (medium protrusion). From the photos of the LB-8, it would appear that the end of the porcelain is either flush with, or slightly recessed from, the end of the plug body (short protrusion). Again, in terms of plug design, and all other things being equal, how does the "protrusion" affect things? There must be a reason why there are different protrusions available. From scrutiny of the Autolite catalog, it does not appear to be heat range related, i.e. similar plugs with different heat ratings have the same protrusion. So...what factor is determined/varied by the protrusion? Bottom line is that while the Atlas 492s are not dimensionally similar to the LB-8, they seem to be a suitable workaround. On the other hand, since there are available cross reference plugs for the LB-8 by both Champion and Autolite, the remaining supplies of original LB-8s should probably be best provided to owners of "show cars" where absolute authenticity is desirable and not to driver cars as in my case where the currently available cross-reference plugs are more suitable. Thanks to all for their input! . | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 1633 Registered: 9-2004 |
Richard Y, maybe two or more, cheers may see you at the 2018 NEC with the cider [somerset]. | ||
| richard george yeaman Grand Master Username: richyrich Post Number: 866 Registered: 4-2012 |
Patrick that would be good if we could meet at the N E C Classic car show there are 12 names down on list be great if you would add yours, Richard. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 1639 Registered: 9-2004 |
Richard, it is a maybe ATM but looks ok so far. Will give you a ring nearer the time. | ||
| Christian S. Hansen Grand Master Username: enquiring_mind Post Number: 633 Registered: 4-2015 |
Are these latest posts somehow in the wrong thread? Seemingly unrelated to small horsepower spark plugs? Have moved photos to a new thread in Idler Chatter titled Our Local Wild Life. |
