ausmini :: View topic - 1275 stroke & bore options

I am looking at doing up a couple of engines for my Twini. 1275's running a single HS6 to maintain that standard look under the bonnet.

What is the best cost effective option for engine capacity, for a fast road car / regular driver?

There are a lot of references to 1380cc, which I assume is a standard stroke 1275 with 73.5mm pistons. How reliable is this for a regularly driven road car, assuming the engine is built correctly?

Do many people “stroke” the big journal 1275 crank? My math indicates that this would give an extra 25cc’s on a 1330 engine going back to standard 1 & 5/8 journals.

What is the cost of such an exercise – machine and re-treat the crank and I assume ease the gearbox housing a bit. Is it worth it?

He reckons the big journal cranks are more likely to crack if left big...

Hi Dr.
What crank have you used?
What journal diameter for the big end?
Why no heat treatment on the crank - nitriding or tuftriding??
Did GR do the crank for you?

drmini in aust wrote:

He reckons the big journal cranks are more likely to crack if left big...

This seems so opposite.....so why are they more likely to crack?

Mine went to 86mm (with 74.7mm Bore) & i still have a few sets of .080" under bearings here & i can now get new ones :-)

(so i`m told)

Author:	IndigoBlueCooperS [ Sun Sep 13, 2009 10:50 am ]
Post subject:	1275 stroke & bore options
I am looking at doing up a couple of engines for my Twini. 1275's running a single HS6 to maintain that standard look under the bonnet. What is the best cost effective option for engine capacity, for a fast road car / regular driver? There are a lot of references to 1380cc, which I assume is a standard stroke 1275 with 73.5mm pistons. How reliable is this for a regularly driven road car, assuming the engine is built correctly? Do many people “stroke” the big journal 1275 crank? My math indicates that this would give an extra 25cc’s on a 1330 engine going back to standard 1 & 5/8 journals. What is the cost of such an exercise – machine and re-treat the crank and I assume ease the gearbox housing a bit. Is it worth it?

Author:	drmini in aust [ Sun Sep 13, 2009 5:26 pm ]
Post subject:	Re: 1275 stroke & bore options
IndigoBlueCooperS wrote: I am looking at doing up a couple of engines for my Twini. 1275's running a single HS6 to maintain that standard look under the bonnet. What is the best cost effective option for engine capacity, for a fast road car / regular driver? There are a lot of references to 1380cc, which I assume is a standard stroke 1275 with 73.5mm pistons. How reliable is this for a regularly driven road car, assuming the engine is built correctly? Do many people “stroke” the big journal 1275 crank? My math indicates that this would give an extra 25cc’s on a 1330 engine going back to standard 1 & 5/8 journals. What is the cost of such an exercise – machine and re-treat the crank and I assume ease the gearbox housing a bit. Is it worth it? A 1380 is reliable but I'd bore to 1360 instead (73.0mm) this leaves you a 1380 bore option for next time. There is no appreciable power gain boring from 1360 to 1380. My stroker motor is 73.0mm bore and is stroked 3.0mm to 84.33mm. This gave me 1412cc. I can't comment on how reliable, it's not in a car yet. It did make 113ft/lbs torque and 113.6HP on GR's engine dyno, with a single HS6. It cost me about $100 extra to get the crank stroked (over a normal regrind). Big journal cranks we got here are soft, no heat treatment needed really. No need to relieve the gearbox, the S rods only swing out by 1.5mm more than with stock stroke. Graham Russell has done 12G1515 big journal cranks either stroked, destroked (to get under 1300cc limit) or just ground down to 1-5/8" journals on standard stroke. He reckons the big journal cranks are more likely to crack if left big...

Author:	IndigoBlueCooperS [ Sun Sep 13, 2009 6:15 pm ]
Post subject:
Hi Dr. What crank have you used? What journal diameter for the big end? Why no heat treatment on the crank - nitriding or tuftriding?? Did GR do the crank for you?

Author:	MG Rocket [ Sun Sep 13, 2009 6:50 pm ]
Post subject:	Re: 1275 stroke & bore options
drmini in aust wrote: He reckons the big journal cranks are more likely to crack if left big... This seems so opposite.....so why are they more likely to crack?

Author:	drmini in aust [ Sun Sep 13, 2009 7:02 pm ]
Post subject:
IndigoBlueCooperS wrote: Hi Dr. What crank have you used? What journal diameter for the big end? Why no heat treatment on the crank - nitriding or tuftriding?? Did GR do the crank for you? I used a 12G1515 crank (late 1100S/Clubby LS) and Cooper S rods. crank was std/std, it's still std/std (but big ends 1.625" now not 1.750"). These big journal cranks (and most Mini cranks except Cooper S) are EN16, not EN40B. Nitriding doesn't work on EN16. The steel must contain vanadium or aluminium to really get any benefit. Like EN40B does. Tufftriding in Australia is now pretty much unobtainable- being a hot cyanide salt bath process, apparently the EPA closed down most shops that were doing it. I hear even the one place in Melbourne that did it for the aircraft industry, doesn't do it now. If you know someone this side of the Tasman that does it, let me know.. Heat treatment (nitriding)info- http://www.heat.com.au/treat-nitriding-update.html GR gave me advice. I got the crank ground by Spiro at St George Crankshafts. He grinds GR's ones. The crank was wedged by me on the mill at home. Then I got GR to balance it.

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Author:	willy [ Sun Sep 13, 2009 7:59 pm ]
Post subject:
The difference in torque is the main thing you will notice. The bigger the stroke, the more torque. As a general rule of thumb! This, combined with an efficient cylinder head and camshaft, and you won't know yourself. Especially with two of the b@stards!!

Author:	drmini in aust [ Sun Sep 13, 2009 9:00 pm ]
Post subject:	Re: 1275 stroke & bore options
MG Rocket wrote: drmini in aust wrote: He reckons the big journal cranks are more likely to crack if left big... This seems so opposite.....so why are they more likely to crack? These big journal cranks do seem to crack more than small journal ones. When Spiro ground mine, he said out of the last 6 of these he got in to grind, 4 had cracks in already. Graham's theory is the BJ crank is too rigid- cannot flex, so it cracks instead. It's just a theory, but there do seem to be a lot of cracked cranks about. IndigoBlueCooperS, the crankpin diameter is reduced by 3.0mm, the crank throw increased by 1.5mm, the stroke (2x the throw) increased by 3.0mm. (for those wondering...)

Author:	GT mowog [ Sun Sep 13, 2009 9:34 pm ]
Post subject:
I read with interest the observations of the BJ Cranks cracking. I have seen a number of these Cranks Cracked or (more usually) in two bits. ALL of these have been caused by previous re-grinds and the fillet radius being too small, or indeed even from the factory (particularly early A+). Not to say that this is the only cause, only the ones that I have seen. I'm not a 'fan' of heat treatment of cranks. It does not make them any stronger, just harder jounals to (suposedly) make them last longer between grinds. It does tend to make the cranks more brittle and if the crank was not stress relieved when originally forged, almost certainly the heat treatment will cause it to bend, which then need to be corrected by bending it back again. Have you even seen how far you need to flex a crank to correct even a small bend? Quite a long way! Not good.

Author:	drmini in aust [ Mon Sep 14, 2009 7:11 am ]
Post subject:
Most of the stroker cranks are just for road cars. Considering these EN16 ones were soft originally, wear life should not be much different. I agree with GT Mowog, the fillet radius is VERY important when grinding cranks..

Author:	The Ranger 89 [ Sat Sep 19, 2009 1:36 am ]
Post subject:
is it feasable to swap the stock 1275 crank out, for a 1098 to acheave more stroke? even with my oversize bores would the crank line up with the bores of the 1275 block? what conrods would you need or would you need to machine the crank? my 1275 is: 72.61 (+.060)bore & 81.28mm Stroke reading a table asphalt posted it shows the 1098 having a longer stroke (83.73mm) aprox 2.44mm more

Author:	drmini in aust [ Sat Sep 19, 2009 7:25 am ]
Post subject:
The main bearings on a transverse 1098 (and all other smallbores) is 1-3/4". All 1275s are 2". So a 1098 one won't fit. (Late 1098 AH Sprites had 2" mains but they are a nth/sth crank). And a 1098 crank is pretty weak compared to a 1275 one. The best bet is get a 12G1505 big journal 1275 crank, and stroke it. I got mine up to 84.33mm, and big end bearing size is std, 1.625". If you grind them undersize during stroking you can get more. Matt Read has some cranks that were offset ground to -.080", I'm not sure of the stroke he got. But I know bearing shells for them are just about unobtainium.

Author:	TheMiniMan [ Sat Sep 19, 2009 4:29 pm ]
Post subject:
Mine went to 86mm (with 74.7mm Bore) & i still have a few sets of .080" under bearings here & i can now get new ones (so i`m told)

Author:	jbeenz [ Sat Sep 19, 2009 4:56 pm ]
Post subject:
TheMiniMan wrote: Mine went to 86mm (with 74.7mm Bore) & i still have a few sets of .080" under bearings here & i can now get new ones (so i`m told) Are you using 2500 pistons? - got any spare, ive got 3 good ones, and one thats out of round...

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