ID this?

[rosscla]

Answers on a postcard....

I turned this up while looking for some other parts today.







This oddball piston came out of a barn find J reg GP in about 1987, the bike itself had stood for a long time so this is thought to be an OLD conversion. The panels were rotted right off and the arch was pretty rotten too. It had an (unsalvageable) electronic style seat and 'supertune' style chevrons on the panels.

The piston is marked 66,4 on the crown and DINAHN 529 under the crown.

It's also shorter than a std piston being about 67mm tall.

Any thoughts on the origin ?

[Adam_Winstone]

Dinamin pistons are quite common aftermarket pistons. This example has been reworked for increased transfer timing, inlet timing and additional oil retention... all old tuning tricks.

[rosscla]

Given what I know of the history of the bike it came out of I'd guess this was done mid 70's does that fit ?

[crooky]

Dinamin pistons are quite common aftermarket pistons. This example has been reworked for increased transfer timing, inlet timing and additional oil retention... all old tuning tricks.

Dinamin pistons are quite common aftermarket pistons. This example has been reworked for increased transfer timing, inlet timing and additional oil retention... all old tuning tricks.

Dinamin pistons are quite common aftermarket pistons. This example has been reworked for increased transfer timing, inlet timing and additional oil retention... all old tuning tricks.

Dinamin pistons are quite common aftermarket pistons. This example has been reworked for increased transfer timing, inlet timing and additional oil retention... all old tuning tricks.

good to see you on here ill resist the temptation to mither you about my gearing issues learned a lot from archived posts on lcgb

[Adam_Winstone]

^... I've been a lurker on here for quite a while but there are enough people on here responding to questions and giving good advice (not that mine is always good!) so there was no real reason for me to do anything more than watch and appreciate what was being posted. It was Richard's original 6V stator thread that made me think that I could help him out... if I could post, therefore signed up Expect me to waste time on both sites now

[Terrible Taff]

Answers on a postcard....

I turned this up while looking for some other parts today.







This oddball piston came out of a barn find J reg GP in about 1987, the bike itself had stood for a long time so this is thought to be an OLD conversion. The panels were rotted right off and the arch was pretty rotten too. It had an (unsalvageable) electronic style seat and 'supertune' style chevrons on the panels.

The piston is marked 66,4 on the crown and DINAHN 529 under the crown.

It's also shorter than a std piston being about 67mm tall.

Any thoughts on the origin ?

My answer is also on LCGB site as you posted it there also!

This is one of the many performance Stage 2 Conversion pistons that were around in the late 60's early 70's. Roy's of Hornchurch had them listed in their Cylinder Conversion Booklet of that time, also I have seen the same piston set up in Arthur Francis 200 & 225's conversions.

The instructions that came with the pistons recomended that the exhaust port was to be raised by 2mm & the bore given a few thou extra clearance due to the higher expansion rate of the piston compared to that of the standard Borgo pistons.

Please note; that the main feature of these pistons was that the crown was machined to advance the transfer timing, lining up with the transfer port top edges that were not always level! The inlet side of the piston was 3mm shorter increasing the inlet period, also oil grooving of the piston below the rings was done to try & improve piston lubrication & reduce seizures! These pistons also cracked from the window cut outs similar to the Borgo piston when using higher rev's!

Many of the engine units of the day were using reamed out SH1/20 carbs @ 21.5mm or SH2/22 reamed to 24mm & the Clubman style exhausts, either a 38mm or 44mm I/D Big Bore systems. Royspeed were the first to develope the 44mm unit along with their special 5 speed gearbox made by Omega in Italy.

Many of the ideas for tuning Lambretta scooters around that time are shown in Ken Herlingshaw's Lambretta Tuning Manual as we Lambretta enthusiasts endevoured to increase the speed & performance of our daily rides.
Very important when attending the weekend events like Sporting Trials, Rallies, Gymkana's etc put on by LCGB, LASCA, EMSA & various larger clubs.
Due to the rivalry between riders & collecting points for where you finished in the events it remained very competative!

I hope that my explaination of how these "Stage 2" Pistons came about & what they were trying to achieve has shed some light on your posted pictures.

Ian Frankland
Taffspeed
07794 003276

[rosscla]

Yes, thanks again, always like to get a spread of opinion, though seems most of the lcgb come on here as well...

[Adam_Winstone]

I like the fact that Frank tells of using a similar method of increasing transfer timing for his Red Devil sprinter, where he actually filled this profile into the crown of the Jap piston that he used. It is suggested in one of the well known 2-stroke tuning manuals that this method be used when determining what transfer timing to use so that you can go back to a new piston if you take it too far, which you can't easily do if you've ground metal from the cylinder.... yet Frank tells of carrying this modification out, then sticking with it and not suffering any related problems.

Same temporary testing theory when shortening of inlet side to experiment with inlet duration.

I remember Stuart Owen telling me a tale of was developing his sprinter and Mr Saxelby told him to dyno his motor as it was, then progressively shorten the inlet skirt and stick it back on the dyno to test again, and to repeat this process to watch the power curve develop until the peak and spread was at its best (noting that just peak BHP alone may not be 'best') and further shortening caused power to drop off. Then simply cut another piston to the length that it was at its best and jobs a good un. When you think about it, piston mods are a pretty good way of developing a motor.

Adam