Piston Sizes

[AlanBartlett]

Piston sizes, looking at getting some pistons for the old orginial block engine, now the size i have measured is 14.209inch, is this about right or am i just measuring wrong?

[george garside]

Allan

Not sure what you are measuring ? the circumference of the piston. Pistons (and presumably pistoffs!) are normally measured by diameter , the 7hp with standard bore size is 2.969" to 2.970". $ rebores are alloews in 10 thou stages with correspondingly larger diameter pistons for each 10 thou increment. The oaversize should be stamped on piston, but not of course on the cylinder bore.

Therefore an accurate measurement of cylinder bore is vital unless you hve all 4 oversizes of piston to play with in which case you caould try a+40 and if this would't go in try a plus 30 and so on.

Cylinder bores & pistons don't wear evenly, tending to wear into an oval shape , being returned to round by reboring. If you havn't got an internal mike or accurate caliper the approx ovality can be established by inserting the largest piston in your possession (without rings) and meaasuring round it with feeler guages.

The recommended clearance between cylinder & piston is with heplex pistons .003" to .0045" or with LAC10 pistons .0035" to .005". (LAC10 pistons can be identified by the letters WA cast inside the skirt.


Just in case you are unknowingly using 8hp cylinders their standard size is 3.0315 " to 3.0325" ( which is virtually the same as a 7hp cylinder overbored by 60 thou .

fitting too small a piston or a round piston in an oval bore will result in a great deal of blowby & loss of performance.

hope this of some help

george

[AlanBartlett]

Hi George, found the original pistons for the old orginial block, but there is no indication as to what size they are, and the top of them is pitted and scratched.

[Keith Andrews]

Alan.. piston sizes bores , rings and even weights are critical to a smooth running, quiet, reliable engine.
I doubt if you have the accurate measuring tools to determine what is what
Tollerances are measured down to the +.- .1000 inch and weights between rods.pistons/gudgens/rings down to the gm
And the gap in the end of the rings are critical
Even honeing what grit u use and the angle of the 'sanding' honing marks is critical to rings bedding in giving good compression etc

I sucgest that you find someone with the measureing tools, a local engine reconditioner or their maybe someone in the club who you can take the bores/pistons etc to and ask them to have a look, measure up etc.

A few scratches and minor pitting is ok...bt no sharp edges as this can cause preignition.

[george garside]

[quote="AlanBartlett"]Hi George, found the original pistons for the old orginial block, but there is no indication as to what size they are, and the top of them is pitted and scratched.[/quote]

try cleaning the top with brillo pad, wire wool or fine emery paper. ovrsize should be stamped on as +10 or whatever or if standard size STD which stands for 'standard' not sexually transmitted disease!. The clearance between cyl & piston is same whatever the oversize aadn is as set out in my previous posting, so without rings put piston in pot & see what size feeler guage will go between them . if it is within tollerences given you should be ok. Keith is quite right about ring gap being important but I canat remember off hand tha required gap. Expect some degree of ovality to piston/bore. This is ok provided the widest gap between piston & pot is within data in my posting. Quite often new rings can work wonders if bore is worn a bit & special rings can be had to give a bit more life without need to rebore.
As keith has said balance of everything is important so make sure both pots/pistons are bored to same size & I think its worth while balancing the cyl head capacities even on a low performance engine (prhaps even more so on a low performance engine s every little helps!) arrange the heads open side upwards on bench, propping them up y some means so that they are dead level in all directions using spiit level. Insert identical spark plugs fully tight and fill each combustion chamber with parrafin using pipette graduated in CC' to establish capacity of each head. If any significant difference the largest capacity head can be surface ground to bring it down to same capacity as other one.

I always used to file the inlet tract on the induction pipe and /or induction tract in cyl casting so that the two allighned perfectly as this is not always so on production pots & induction pipes there aften being some sort of step. this can be worked out by putting a thin layr of blutack round the orifice, bolting the 2 parts together lightly and then unbolting so you can seee the marks where they overlap on the bluetac. could probably be done with thin cardboard or maybe gasket paper.

none of above is necessary if its for demonstration build up engine & even fit of piston in cyl is not relevent unless you plan to start up after build up.

If thats the case leave absolutely nothing to chance i.e. build up and start up prior to the event , then very carefully dismantle considering carefully where you can get away with a bit of cheating eg leave valves already fitted & ground in , have distributor on dynamo, have oil pump fitted,- have pistons complete with rings fitted to conrods. have carb fitted to induction pipe. if unable to do previous start up/strip down just settle for the build up on the day.

[Keith Andrews]

CC ing the heads, if u dont have a pipette, head up to the local chemist shop, get a calibrated syringe...tell him u dont need the needle part..
In NZ there are some laws about supply of syringes, dont know what they are, but I use them for measuring chemicals for the gardens etc
And used one to cc the new Bradford Engine heads.
Which reminds me I must re check since they have been planned..hopefully my calculations are right

[george garside]

Keith
perhaps you could post the 'calculations' needed. Its a long time since I've done it so can't be sure of getting them correct, your version may well be of help to people.
george

[Leo Bolter]

Hello all.

Reading this thread as it progressed, I realised that I just happen to have pictures of the setup I used when checking the cylinder head capacities when I overhauled a Javelin engine back in 1998 . . . . I thought they may help to illustrate the procedure as outlined by others and as I used to do that job.

Scrounge (or even buy) a suitable sized piece of clear Perspex (Acryic) or glass and have about a 6 to 8 mm hole made in it.

With an OHV head fit the ground in valves and springs to the head, of course.

Set the head up level. I usually use a piece of round bar stock that's been welded to the main body of a spark plug (with the porcelain smashed out!) with it threaded into the spark plug hole to securely mount the head in a vice. That way it can be easily levelled and is going to stay that way while you work on the job! You may want to fill the interior of the ex spark plug body with something like blu-tac, putty or RTV rubber so as to accurately measure the volume of the combustion chamber.

Grease the gasket face of the head and press the clear plate firmly down on it.

Measure the amount contained in the combustion chamber by carefully trickling the kerosene into the void via the hole until the bottom of the hole is reached.
Small bubbles may be trapped, but they can usually be encouraged to the hole by careful tilting or/and pressure on the "Perspex" window.
(A tip: Put the outlet of the pipette through the hole almost to the surface of the head until the void is full, then withdraw it from the hole to finally 'top it up". This is so as to minimise stray bubbles!).



THE SET-UP (Notice the external balance pipe fitting in the carburettor adaptor!)



A CLOSER VIEW

[Keith Andrews]

On Bradford, and I rhink pre war much of the combustion chamber is also built into the bores. Javelin and Juypitor I dont know about.
Checking these is a bit more fiddly but much the same principle
Mount the engine bore, then turn on its side and as above level.
Turn the crank so the piston is over 1/2 way up
A very light smear of grease on the bore to seal the rings
Then turn the pistion to TDC and measure volume as above.

Repeating with the piston BDC one can determine compression ratio
Bradfords and I think prewar have square cam proftles (valves open close damn near on TDC and BDC so static compession and dynamic comptession are close to the same

English Cars upto 1953/4 where designed to run on low 70 octane fuels and very low compression. Running modern 91 ocatane reduces the economy and power of the orginal cars.
To regain this power one can deck the bores and plane the heads considerably...my bradford heads I have taken over 3mm off them without any interferance between valves and heads measured without gaskets on.

On engines running higher compression , the greatest crank failure risk is not because of the compression, but not having the bores/heads the same.

The other alternative is to run a low octane fuel...A 15% A1 jetfuel and 91 octane gives a octane around 75 to 78.
A1 jet fuel is not the same as kerosine, it is more highly refined, has anti gummng agents, lubricants.
Kero is ok if the engine will be working hard all day, such as a tractor ploughing a feild, but dont use it to drive the tractor home.