6 replies to this topic

[100ev8]

im looking in to building a short stroke mini engine and wonder if anyone got any tips ?
best crank to use ? rods ? block ? pistons etc.
all parts must be readily available if poss and not expensive of the shelf parts
any help will be much appreciated and welcome

rob

[Wil_h]

all parts must be readily available if poss and not expensive of the shelf parts

Then forget building a short stroke motor.

Cheapest way is a 1070 (or SA) crank in a 1275 block suitably decked. But 1070 or SA cranck are neither sheap of readily available.

[Paul Wiginton]

Short stroke will rev superb, but it will not have as much power as a normal stroke even with the same cc.

[miniobsessed]

You could offset grind a 'large journal' crank the opposite way to most people and reduce the stroke. Then run 'S' rods. Would give you a stroke of around 78mm.

Or you could do something similar to a guy here in Oz (he's a bloody genius!)... He's modifying a 1275 block to accept a 998 crank which will give a 76mm stroke (bike head project).

If this is for a road engine (even if it's for a competition engine) you're really taking a backward step by reducing the stroke as it will have a huge impact on the amount of torque the engine will have (and at what point in the rev range your peak torque comes on). And even if you reduce the stroke dramatically you'll be hard pressed to get anything 'usable' past about 8500rpm with any sort of reliability - the 3 journal crank is just really not up to it.

Even with it's 'long stroke' a 1275 can be useful right up to 8500rpm if you do all the right things to it and are happy for it to go bang eventually (fatigue). Keep it below 7000rpm and it'll (just about) last forever.

[Wil_h]

Thats a funning big job, the crank is shorter and has smaller journals. For the cost it'd cheaper to buy a billet short stroke crank.

Edited by Wil_h, 27 August 2010 - 04:29 PM.

[miniobsessed]

Thats a funning big job, the crank is shorter and has smaller journals. For the cost it'd cheaper to buy a billet short stroke crank.

He's a semi-retired engineer doing all of his own work so cost is not really an object... I would argue that modifying the block would be a once off job and there are millions of standard 998 cranks out there (if it went bang) instead of paying big money to produce a one off crank.

[Wil_h]

Well there is some logic there. But in 10 years of racing I have gone through 3 blocks, but I'm still on the first crank.