Hitchcocks Forum

Hi all 612 owners,
What thickness compression plates are you using with the 8.5:1 forged piston? The kit is supplied with a 2mm & 3mm plate and 2 base gaskets. The long stroke crank has a +13mm stroke to standard which suggests 6.5mm of base compression plates and gaskets to maintain the same compression? Does anyone know what compression the 3mm plate gives with 2 gaskets (4.7mm thick) and a composite head gasket? (1.3 thick) Is there any proplems with head / barrel stud length using the compression plates? Thanks for your help

The long stroke crank has a stroke of 103mm, compared to the standard stroke of 90mm. The difference of 13mm would mean that with a standard conrod, the piston would travel 6.5mm lower down and 6.5mm higher. But the conrod on the longstroke crank is 3mm shorter than standard, so the piston in actual fact, reaches 3.5mm higher at TDC. So a 3mm packing plate under the barrel, plus two base gaskets, gives you the 3.5mm you need to bring the piston height back to what it would be if you put the 8.5 compression Acralite into a standard engine. However, I have found that my engine is prone to pinking when accelerating hard with just the 3mm packing plate and 3 base gaskets fitted (one extra for luck!), despite a fairly retarded ignition setting (approx 26 degrees BTC, fully advanced) and comparatively rich carb settings (a 107 needle jet set on the middle clip position and a 260 main jet). However, I have now fitted a 300 main jet and use BP Ultimate high-octane petrol, along with a drop of “Castrol Valvemaster Plus with octane-booster” and I have no more pinking. If I was building the motor from scratch again, and having the benefit of this experience, I would have gone for two 2mm plates and three gaskets as an absolute minimum, in an attempt to combat the pinking. Having said all that, I must say that my bike goes like hell now and I am very pleased with it! But no two engines are the same, and this is just my own take on it for what it’s worth. On the compression, I haven’t the foggiest as I haven’t measured it, and the standard cylinder studs are fine (sufficient threaded length) and I torqued the head down to 19 ft/lbs and re-torqued to that figure after a bedding-in run (when cold). Hope that helps!

I don't have a 612, but a 625. I fitted a 3mm compression plate, and it pinked. I retarded the ignition and it still pinked. then I fitted a pair of our hosts' advance/retard springs, which are stronger than the floppy Indian originals. Result - no more pinking. The new springs prevented the ignition reaching full advance too quickly, and transformed the running. Just a thought.............

I'm writing this info down as it's going to important when I get the long stroke crank .

Thanks for your help and quick response. I have run a 535 for a short time at 8.5:1 using valve master and Boyer micro-digital ignition with no pinking at all. Our host suggested that the improved advance curve helps over the standard set up but I was always worrying about the bottom end failing hence the long stroke crank. That said the engine strip revealed no horrors and sound bearings(still going to change them)but please don't ask me to justify the expense! Think I'll try the 3mm plate and two thick base gaskets (The gaskets provided as a set are half the thickness of those ordered separately) Cheers.

Hi "another Allan" - I remember your bike when you bought it on ebay. I thought at the time that it was a cracking bike for somebody! I did fit our host's stronger a/r springs when I built my motor, but it still pinked as I described. It seems fine now though, with the timing a gnat's todger on the retarded side (about 26 degrees) and the 107 needle jet/300 main jet. I have a hunch (which may be unfounded) that the very sharp edges around the valve pockets of the Acralite might be a factor in the pinking scenario, so if I ever get around to stripping it, as well as giving another millimeter of barrel packing I will take a file to those sharp edges!