BrianJT Posted May 21, 2014 Share Posted May 21, 2014 Hi, I've just been told by a mechanic that my spark plugs (Champion N5, 3/4 inch reach) are too long for the head on my engine. The basis for that claim is a clear combustion colouration for the last 3/8 inch nearest to tip. States that correct reach is 3/8 inch. Any comments (apart from 'Well change them, then') as to this possibility? Has it to do with different compression ratios - my car manual states that optional ratios (instead of 8.5:1) of 9.0 or 9.5 are/were available. The implication is that the 3/4 inch reach plugs were meant to be used with the 8.5 ratio but does that agree with the reduced combustion chamber volume that would result from having a greater thickness of chamber wall?All constructive comments will be acknowledged with gratitude.BrianJT Quote Link to comment Share on other sites More sharing options...
dinger Posted May 21, 2014 Share Posted May 21, 2014 Take a plug out and shine a torch down the thread to check . Bend a piece of wire and use it to chack thread depth .. I thought that all Triumph engines had 'long reach' plugs fitted . Quote Link to comment Share on other sites More sharing options...
herald948 Posted May 21, 2014 Share Posted May 21, 2014 As I recall, the 948s did use a 3/4" reach plug, while 1147s had a 1/2" reach plug. It went back to 3/4" reach for the 1296. You might look for the stamped part number on your head; perhaps it is from an 1147? Quote Link to comment Share on other sites More sharing options...
heraldcoupe Posted May 21, 2014 Share Posted May 21, 2014 herald948 wrote:As I recall, the 948s did use a 3/4" reach plug, while 1147s had a 1/2" reach plug. It went back to 3/4" reach for the 1296. You might look for the stamped part number on your head; perhaps it is from an 1147?3/4" is correct, though plenty of literature down the years has (incorrectly) stated the 948 plug length as 1/2".The 1200 head will fit straight onto a 948 block so it's not inconceivable that the head is the wrong type,Cheers,Bill. Quote Link to comment Share on other sites More sharing options...
BrianJT Posted May 22, 2014 Author Share Posted May 22, 2014 Hi Bill, Andy and Dinger,I'm grateful for your very informative comments - thanks a lot!Brian Quote Link to comment Share on other sites More sharing options...
BrianJT Posted May 22, 2014 Author Share Posted May 22, 2014 Hi Bill, If the head is of the wrong type, would this prevent the engine from running or simply have an effect on its performance? Can you direct me to the position of the stamped part number? Quote Link to comment Share on other sites More sharing options...
heraldcoupe Posted May 22, 2014 Share Posted May 22, 2014 The 1200 head would lower the compression substantially, but I can't see any reason why it wouldn't work. With some work the compression could be brought back up, while everything would line-up OK.I don't have any heads around or decent library photographs, from memory the 948 head has markings cast in the top face, a rather prominent "10" comes to mind,Cheers,Bill. Quote Link to comment Share on other sites More sharing options...
Casper Posted May 23, 2014 Share Posted May 23, 2014 1200 stamping is on the flat machined face where the rocker cover gasket sits (may be partly covered by the rocker cover). I seem to recall somewhere at the rear, but can't remember which side.1200 heads include:510461 8:1514431 8:1514441 8.5:1513963 8.5:1 (also 510462 and 514432 6.8:1)The CR applies of course if fitted to a 1200 engine. Quick (rough) sums suggest that a 1200 8:1 head on a 948 would give just under 7:1 and a 8.5:1 head just over 7.25.C Quote Link to comment Share on other sites More sharing options...
BrianJT Posted May 25, 2014 Author Share Posted May 25, 2014 Hi Bill and Casper,Thank you for the replies - it's good to get such help!.Regards,Brian Quote Link to comment Share on other sites More sharing options...
logicaluk Posted May 29, 2014 Share Posted May 29, 2014 Casper what about head no 50989 I think its 1200 8:1 low c but im not 100% sureDan Quote Link to comment Share on other sites More sharing options...
Casper Posted May 29, 2014 Share Posted May 29, 2014 Sorry, Dan.I can't help on that one.C. Quote Link to comment Share on other sites More sharing options...
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