May 31, 201412 yr Can any one please confirm the correct NGK Plug for a 1979 Montesa 247 Cota I seem to get either a B5HS or a B6HS ??? Any thoughts on an iridium tipped plug in it ?? Cheers for now As Always The Wobbler
May 31, 201412 yr Author sorry just to clarify I have a BP5ES in at the minute but on NGK 's web site it suggests a B6HS for a 247 and a BP5ES for a 248 - it has always had a BP5ES in which I think is a slightly longer reach ?? Any help is appreciated As always The Wobbler
May 31, 201412 yr I had a peek on the NGK site at the plug decipher chart... Lets take the BP5ES.. B is for thread size, B = 14mm thread, 20mm Hex. P is for Projecting insulator type, to me that means the insulator projects past the plug base. 5 is the heat range, goes from 2 to 10, 10 being cold, 2 being hot. E is the thread length, E = 19.0 mm S = Standard Type Plug. Now the B6HS B = thread size of 14mm 6 = One range colder than a 5. H = thread length of 12.7 mm S = Standard type plug. The heat range is close but in general the hotter plug will make the engine run hotter but it is fraught with danger going too far from factory recommended range… see here ...http://www.ngk.com.au/spark-plugs/technical-information/heat-range-explanation The main issue I see above is the thread length… you don't want threads protruding into the cylinder the thin edges can cause pre-ignition if they get too hot. One has a thread length of 19 mm verses the other at 12.7 mm. That means the 19mm plug may protrude 6 mm or so in to cylinder if head is only 13 mm? Check that with a torch… that may give you an answer as to the right thread length for your head???? Mags
June 1, 201412 yr Hi I have used a B6HS without any problems for the past two years on my 1969 247, both on points and Electrex world ignition systems. Regards Martin
June 1, 201412 yr My 1975 Cota mk4b uses B6HS, and I also have a B5HS in the position where the decompressor should be instead of a blanking plug, I can use this plug when the weather is hot so that the engine can run a bit cooler and it's also an instant backup should the main plug foul ! As for Iridium plugs, why pay 4x the amount for a plug that is only designed to last between the increasing service intervals of modern cars! A two stroke trials bike will need a new plug roughly 2/3 times a season due to carbon buildup and at a couple of quid a shot they are dead cheap and always the first thing to change should any electrical problem/ missfire occur. Cheers Dave
June 2, 201412 yr Author Gents, Thanks for the info - my confusion came from the fact that its been running with a B5PES which has a slightly longer reach. it all seems OK and I just wondered what effect the shorter(correct) plug might have (As these also fit my 240 Fantic). My bike is one of the very last 247's 1979 and I don't know if they differed from the early ones..... Triple X glad to know the electronic ignition is going OK ....You must let me know what Sammy Miller & Twinshock Championships you're doing as I'd like to have a go & check the difference... Cheers as always The Wobbler
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