25 replies to this topic

[DeadSquare]

As 1600 Triumph Vitesse plus .060" pistons are not available, can anyone here offer advice / experience about reducing the diameter of an A series 1275 piston?

[GraemeC]

I would suspect it is a lot of work (if possible given they'r not fully parallel sided and you'd need to deepen the ring grooves and then find suitable rings.

 

Can the Triumph engine not be linered?

[DeadSquare]

I have a lathe, and a modicum of skill, so I can turn the crown and deepen the grooves, and I would think that the rings could be re-gapped.

 

I am wanting the pistons for an 850 Mini.

[mini13]

Nit really worth the effort, as you'll end up with a weak skirt, and probably ring groves that are not accurate enough. Also pistons are not round, they are tapered and barrel shaped and often oval, basically they are shaped so that they ate the right shape at running temperature, best of just to get some modern ones, do it right, do it once

[DeadSquare]

Where do I get "some modern ones" that are 68.25MM diameter,  1 1/2" compression height and either .624" or.8124" gudgeon pin ?

[whistler]

Seem to remember Hillman Imp pistons being similar size.

[DeadSquare]

Thanks for the suggestion.

 

The Imp compression height is 1,05", so about 1/2"  down the bore at TDC, and although the standard bore is 68MM, their oversize are .15MM / .3MM and .45MM; but  no 68.25MM.

 

Simca also used to do a 68.25MM piston with about 1 1/2" compression height, but they were in a 1950's engine.

[mini13]

hang on, 850's were a smaller bore werrnt they? 62.9mm?

 

also what year block? as iirc the early ones didnt have much meat for boring. but you could maybe use 998 pistons and deck the block a load.

[DeadSquare]

Yes, 850's are 62.9MM.

 

I can cope with there not being much "meat".  By using Fiat liners and boring the block off center it lines up with Cooper S rods.

 

A  998cc has a bore of only 64.588MM, so plus .100"  would only bring the bore to 67,138MM.

Edited by DeadSquare, 29 October 2018 - 12:04 PM.

[KTS]

..if you haven't figured out where this is going, the clue's in the name..

[nicklouse]

what did AC Dodd use in the biggest little build he did a few months back?

[ACDodd]

Why on earth would you want a square engine?

Ac

[DeadSquare]

You may wonder why someone would want a square engine "on earth", but in the sky, Henry Royce's favourite engine was the Kestrel, produced at the end of the 1920's

 

It was conceived as a square engine, because that gave the lowest piston speed for the best cylinder filling with the highest torque.

 

I have been told, but can't prove, that all the RR bird of prey engines were developed as square engines, but to comply with ministry requirements for a reliability margin, had the bore reduced by 5% for production.

[Retroman]

68mm pistons are available from Calver ST

 

I have used them in both 998 and 1098 the overbore is 0.135"

 

Works a treat, 1107 cc's from a 998 made over 80 hp and 60lb+ of torque from 3 to 6k   [ 270 cam 940 head & vizarded HIF 38]

[DeadSquare]

68mm pistons are available from Calver ST

 

I have used them in both 998 and 1098 the overbore is 0.135"

 

Works a treat, 1107 cc's from a 998 made over 80 hp and 60lb+ of torque from 3 to 6k   [ 270 cam 940 head & vizarded HIF 38]

 

 

Thanks, I contacted him about a year ago and didn't get a coherent reply.

 

What was the compression height of the pistons that you used, and the gudgeon pin diameter?