Pistons with 1/2" gudgeon pin

[Captk]

I am interested in getting some advice on pistons. I am rebuilding a 1914 9hp Thor. The original cast iron pistons are not re-useable and the bores needed a clean-up. I can get pistons made here in Australia, no problem, but I have two choices. JP Pistons can do the right dimensions in a cast aluminium piston but the smallest gudgeon (wrist pin) size they can do is 3/4", this is dictated by the dies they have available. Thor are 1/2" pins. Specialised pistons can do the pistons in a forged aluminium at about three times the price, but can use the original pin size. I have the con-rods already reconditioned with new bushes and are ready to go. One choice is to get the cast pistons and bush the piston to suitthe 1/2" pins. Or should I just get the forged items?
Anyone know of off the shelf pistons with 1/2" pins and a 91mm bore?

Kelvin

[kg993]

I can't help with off the shelf pistons but curious what they quoted you to make them. Aris just quoted $500.00 to make 2 +.020 and 2 +.025 pistons including rings and pins. It's about the same price to have just 2 made. I think thats a pretty good deal.

[Captk]

Didnt get a price on the cast pistons but going by past experience eith them it would be under $400/pair with rings, gudgeons and clips. The forged ones are quoted at $1100/pair (complete) or $1400 if i get 4 all the same size. I am not too worried about the price, more interested in the technical opinions to do with cast versus forged pistons in a old bike.

[sirhrmechanic]

Try Arias pistons in California. They can do anything at all for you and their prices are a lot less than you are being quoted for the Forged.

If getting them done from Australia is a problem, let me know and I'll be glad to work with them to get you a set done. Or give them a call and get extension '5' and talk to Nancy. She will take great care of you.

JP make good pistons BTW. But you probably don't want to bush them. Too much going on... to many risks. Get them made to a right fit. You don't want to lock up a piston wrist pin or scuff one and air cooled aluminum pistons are not to be trifled with... they're delicate enough to begin with.

We now have Arias make pistons for us for prewar British cars and they are doing superb. We started using them w. Vintage Bentley's and now use them virtually across the board.

Let me know if you want some help and glad to do it.

Cheers, Sirhr.

[Captk]

Thanks for the offer Sirhr, I have decided to go with the Spec. Pistons from Dandenong in Victoria. I think I can match all four barrells from both of my engines to the same size and keep cost down to around $1400. They offer a slippery coating for an extra $30/piston, which I will probably do as well. Any thoughts on rings? It seems they oriinally had two wide compression rings, but someone has added one oil ring. I have seen some total loss V twins with one piston with just compression rings and the other with a scraper as well??

[sirhrmechanic]

Capt... rings are always a quandary, especially with really early bikes... and I don't have a great answer for you. Certainly not one from experience on an air-cooled V where the cylinders can be running very different temps, etc. Even using aluminum pistons in some of these early bikes seems a bit scary, just because they are so delicate and susceptible to scuffing or galling or other nastiness.

Here are a couple of thoughts... but chalk this up to 'thought experiment' not to real advice. But something to consider.... hopefully someone with more direct experience on the early V-Twins will step in. That bike probably has 'splash' lubrication. Total loss... it's oiled by getting oil flung all around the inside of the engine casing by the flywheel and con rod(s). There is no strong spray, no 'extra oil valve' no oil coming out of .015" pressure holes in connecting rods (as is the case with some of the cars we work on)... So whatever oil is on the cylinder wall gets there in mist and odd droplets flung around the engine. I also presume that there is no 'sump' or anything from which oil dippers are working.

In such an environment, I would think one would want to keep the cylinder walls pretty wet... and if you have an efficient oil scraper ring... do you risk having insufficient oil to keep the piston from scuffing?

I'd also wonder what is the piston clearance? Those pistons are going to expand many X more than the cylinder bores when it gets hot. Can you get away with an oil scraper if you use a bit more clearance?

I don't know the answers... these are just some thoughts. But were I working on some piston ideas, these are the questions I would be asking myself.

Overall, these are also the questions I'd be asking my piston maker. I figure piston makers are a lot smarter than me... especially when it comes to making pistons. So they should be able to give you an application-based recommendation of style of motor, type of lubrication, compression ratio, valve configuration... then be able to recommend what alloy, style (forged solid skirt or T-Slot?), what cam-grind on the piston, number of rings, Ring material, oil scraper yes/no, etc. etc. etc.

They should be able to make the right recommendation... and if they can't, I'd be wondering why I chose them to make the piston! ;-) Pistons can literally make or break an engine. Approach 'custom' applications with caution. Especially when replacing cast-iron with aluminum in early machines. Cast iron pistons are unbelievably forgiving. They put up with anything. Aluminum pistons are very temperamental. One little thing and... ba-Bang-o.

Cheers,

Sirhr

[Captk]

Thanks again Sirhr,
Love to hear from anyone that has gone down this path, I am sure I am not the first in this group!
I would love to have kept using the CI pistons, but not possible on my project. It seems only aluminium pistons are made these days so that kinda limits that option. On the question of clearances, yes they do recommend tolerances that I will go with, probably in the region of 0.005/6"
Generally they want to fit oil scraper rings, but I am unsure. I tend towards your thought on too much oil is best for these early engines. My early Brit bikes all call for a thin blue haze from the exhaust.
Being total loss must mean the oil needs to be burnt to get rid of it in between draining the cases periodically?

Kelvin

[Captk]

Thanks again Sirhr,
Love to hear from anyone that has gone down this path, I am sure I am not the first in this group!
I would love to have kept using the CI pistons, but not possible on my project. It seems only aluminium pistons are made these days so that kinda limits that option. On the question of clearances, yes they do recommend tolerances that I will go with, probably in the region of 0.005/6"
Generally they want to fit oil scraper rings, but I am unsure. I tend towards your thought on too much oil is best for these early engines. My early Brit bikes all call for a thin blue haze from the exhaust.
Being total loss must mean the oil needs to be burnt to get rid of it in between draining the cases periodically?

Kelvin

[sirhrmechanic]

Correct... the oil burns or drains...

In terms of CI pistons... it is possible to buy cast iron 'slugs'. We use them all the time in manufacturing 'stuff.' I've never made a piston set, but it would seem to me that any piston maker that can make you a pair of pistons from a forged billet... should be able to program the machines for cast iron. Maybe not cost effective for one set, but I would think that for bikes like JD's or early Indians, where lots of them are running around, that someone could do cast iron pistons as customs.

I can check with Nancy at Arias if it interest folks. They may not make them... but maybe they know who can/does/would?

Personally, for early machines, I am a huge fan of cast iron pistons. By way of illustration... 3 years ago riding back from an informal show, my friend Fred forgot to pump the extra oil 'squirter' on his JD on a long hill. At the top of the hill, the engine coughed and locked up solid. I was sure he had toasted it. He said 'nah,' we waited 10 minutes and he kicked it and away we went. The original HD cast iron pistons he is running simply cooled, loosened back up and he was away. Admittedly, his engine is not 'break-in' tight, but it's a brilliantly sorted machine that he rides everywhere. So pistons are not rattling around the bores loose either. If those had been aluminum pistons, that engine would have needed a whole new top-end. Cast iron pistons are very forgiving in early machines.

Cheers,

Sirhr