Hi its been a while since last post but ive been busy restoring a pinball and some watches. I am having some major issue with my BSA Firebird with SRM 750 conversion. I cant get it passed the 500 mile mark without it pinching up the left side piston, 3 times now. the engine tightens up to the point of seizure leaving a flat spot on the lower back of the piston. I have two barrels both professionally bored and it has happened on both. Piston clearance is correct and have had two different piston manufactures SRM and JP. OIl pressure appears correct. Could this be a result of a slightly bent rod or any other ideas please. Thanks Mark
The way things are going i too might have to sell for $50000 like the eBay T-Bolt
i would give it more than 5 thou,i had a 500 triumph(69 bore ) and that needed 6 thou even with geniune powermax pistons,the bible tuning for speed says between 2 to 3 thou per inch of bore,sprint engines have as much as 20 thou so i would go about 6 1/2 with 79mm bores
Boyer ignition 32 deg btdc, 32mm Concentric carbs, slide 3,needle jet 106, needle bottom notch, main jet 190 i think, we were having trouble with fuel and sooting up plugs, flowed head, 9-1 comp, 43.5 mm inlet valves, high output oil pump, roller bearing conversion, srm pressure release valve
What part of NSW Mark? Anywhere near me? When you say lower back of the piston your talking about the skirt where it runs on the centre of the bore facing the rear I expect? If the rod is bent one would expect the wear to be on the side. If your motor has a roller conversion check that the position of the crank puts the rods central in the bores. What did JP recommend for piston clearence in an iron cyl? I've used JP B44 pistons in an alloy block and they were fine. The best pistons and rings I've used are JE forged B44 from Ed V. They can run quite tight even in an iron cyl.
Ok- a genuine measured 9:1 is highish on a 750 Brit twin nowadays, but maybe ok. Measuring compression ratio with a burette is the sort of thing you have to do if you build a a tuned oversize special, which is what you have done, no matter what "bolt-on" claims you may have heard from sellers of big bore kits, etc.
I'm assuming it's an iron barrel. Wak has gone to the trouble of looking up Irving's recommendations and it looks like you're well under his minimum. I'm using 5 thou clearance on an iron headed Triumph with 72.5 mm bore. It seems to work well, but it's no racer although it runs hot as bloody Hell. Once again, you have a souped-up bike which may need to be on the loose side of recommended clearance.
At the end of the day, you used up all the clearance you had, when it seized. Honing one of your barrels out sounds like it's worth a try.
190 seems too small for a main jet, based on using a bigger carb and incresing the bore I would have started at 230 to get it running rich and then worked my way down. Just because the plugs are fouling just says at one particular throttle opening you are too rich. So you could be too rich just at 1/4 throttle, ok at 1/2 and lean from 1/2 upwards.
You've to credit him with enough sense to use carburettor settings derived from his own road or dyno testing.
When doing trouble shooting leave no stone unturned, I always look at what I did or did not do which I should first as I am normally the culprit. Realising it was my cock up is preferable to not finding the problem. Its not a blame game.
Thanks guys for the input. The 190 main do seem low but we worked down from 220 (Road). As for the oil hole in the rod i cant remember, i do have a set of new rods somewhere in the shed and i know one has the oilway.
Needle clip in lowest groove will give rich running this may have confused main jet selection by making the midrange over rich and helped wash oil off the bores leading to the nip ups. What oil are you using?
Last edited by gavin eisler; 03/18/126:05 pm.
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
Here's a total shot in the fark but I haven't seeen it mentioned .Is it possible the piston skirts need to be clearanced ?Are there any witness marks in this area on the bad pistons?as the engine is assembled this can look fine but with revs the crank can flex and do some silly stuff .
Marl: The piston and wrist pin will often give up a lot of information about the cause of the seizure. We are not 16 again, "Working on mysteries without any clue."
The question is, did the piston seize from:
A Lack of Lubrication. Area of piston seizure is constrained to the bottom edge of the thrust faces of the piston. Melted aluminum will be smeared UP across the thrust face. A little bit of pin discoloration. No burnt carbon underneath piston dome.
Lack of Clearance. Area of seizure constrained to area just under oil ring. Melted aluminum smeared DOWN the thrust face of the piston. Very little pin discoloration. No burnt carbon underneath piston dome. Area under opposite side of seizure show signs of polishing - aluminum has started to melt.
Too much heat. Dome of pistons show signs of detonation - aluminum pistons sparkles through burnt on carbon. Carbon burnt on under piston dome often flaking. Pin discolored from dark blue to purple. Seizure on four corners of piston just for and aft pin hole. Often seizure continues to area under oil ring.
and the prize goes to............................ LACK OF CLEARANCE Area of seizure constrained to area just under oil ring. Melted aluminum smeared DOWN the thrust face of the piston. Very little pin discoloration.
Unless you are 100% certain of your mixture, particularly on the needle and main, you're asking for trouble. Particularly the needle mixture is oft ignored, despite the fact that this is where you spend most of your time. If you're lean on the needle, it will run hot and it will seize. Ask me how I know...
A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.
Alex: When you have that kind of heat seizure the oil will bake underneath the piston dome. The heat will migrate down the piston pin support casting on either side heating the pin. The temperature the pin reached, that caused the seizure, will be recorded in the color of the pin: Light Straw through dark purple into black.
Seizures, where the pin is still silver as it came out of the box and there is no burnt on carbon under the piston dome, is telling you the piston was within normal operating temperature and regular piston expansion caused the seizure.
Income: The lack of oil seizure is distinctive in that the seizing is at the bottom of the bore. Yes, if the crank is plugged with crap and oil isn't flowing, you can get a piston seizure.
Reading pistons for failure analysis is a bit subjective. Especially because a lot of references are for diesel, two stroke and water cooled engines. But there is a lot to see in a damaged piston and with a little common sense you should be able to sort out the cause. A lot of heat can only come from combustion. Gotta go... more later...
What do people think of these cut away pistons they do seem heavy with thick walls they are from JP pistons AU, should i clean them up and reuse or full replace. Cheers Mark The blue color around the pins is a reflection
Yes, clean them up. However I would go one further and machine the siezed piston so that it is back to the same profile as the other ( might be oval & not round ). Take some better photos. Turn off the flash, put your camera on a stand of some sort, measure the distance from the piston to the lens so that it is in clean focus then send them to JP asking politely for their recommendations. They are usually more than willing to help when asked nicely. JP's usually run with a big clearance. Also carefully check your bore. Easiest way is with a ring sitting on the bottom of an upturned piston to keep it square in the bore. Light it from underneath and look for a consistent ring gap @ 1/4" intervals and also for light any where between the ring & the cylinder wall. Ignore this if you have a lathe or mill and a good tickler gauge to check the bore with. A lot of machine shops mount the cylinder from the head ( because that is the way cars are done ) in stead of the flange so you end up with a bore that is true to the sloping head and skewed to the crank.