What piston?

Caddy · Post by **Caddy** » Sat Apr 02, 2011 10:02 am

Good morning all,

Can anyone help? I'm looking to increase the compression ratio of my 350 H/W based v twin and need a little help in sourcing a couple of pistons to do this job. Are there any suggestions? I will be boring (everyone?) out both cylinders so that they are both the same diameter. I wondered if it was worth looking at pistons from other makes......

john · Post by **john** » Sun Apr 03, 2011 12:07 am

Wait, let’s rewind a little here,

You are making a 700cc V twin from two 350 singles and you are asking us for advice?

With standard pistons you can go up to 9.9:1 CR on a hepolite scramble piston but with your skill set you could just shorten the barrel to give you whatever CR you wanted. The ultimate on standard pistons would be to fit a pair of short stroke pistons and get a 820cc twin.

But tell us more about the project, how are over coming thing like oil pump and cam operation?

wilko · Post by **wilko** » Sun Apr 03, 2011 5:30 am

There's a new high comp piston on Oz ebay at the moment. Maybe he has another? Although it looks a bit radical!!

Caddy · Post by **Caddy** » Sun Apr 03, 2011 10:53 am

Okay, a quick rewind. I wan't a pre unit V twin engine to put in a wideline featherbed frame a Vee-Ton if you will. Problem is a new JAP will set you back £14,400 plus. I have just restored a brace of 350 Jampots and have an engine left over plus two cylinders and heads.
A few drawings later and the concept of an AJS OHV V2 was born.
Oil pump will be from an SRM BSA A10 pump, ignition from Pazon, and cams etc are going to be reasonably easy to get made. Just got to design the crankcase and get a decent amount of compression without having too much of a domed piston (plays havoc with the combustion process).
Oh bottom end will be loosely based on a hybrid between a G3 and X2 engine.

Caddy · Post by **Caddy** » Sun Apr 03, 2011 12:18 pm

Just a quick question regarding the short stroke piston; the only one I can find is for a 500cc engine, 86mm dia. Can a G3 cylinder be bored out by 16mm?

john · Post by **john** » Sun Apr 03, 2011 4:00 pm

The 1964 road 350 had a 74mm bore, using +040” thou pistin (030181) in a long stroke trails engine was a popular period mod.

There was some talk on the forum about getting a batch of these pistons made, with a forum search you may find that they are easily available.

I am very interested in the details of your project; I hope you keep us posted

Caddy · Post by **Caddy** » Wed Apr 06, 2011 3:31 pm

Well I might have found a pair of 74mm pistons. Do you need to use the +.040" pistons?
As I'm going to design a bespoke crankcase, and the conrods will be bespoke I can adjust the piston to fit perfectly.

Now how much work can be done on the head? I really shouldn't have read 'tuning for speed' by P E Irving.....
John, you asked about the cams; I'm going to get a pair of piggy back cams made up using the SH profile. I've found out who Jampot Spares have asked to make some new sets up and I'm speaking to them about that. I've also been thinking about making a OHC rocker box.......
.... Might be taking it too far though.