ausmini
https://ausmini.com/forums/

How to build a big bore stroker donk
https://ausmini.com/forums/viewtopic.php?f=5&t=6849		 Page 1 of 4
Author:  Metalfab_101 [ Tue May 03, 2005 7:59 am ]
Post subject:  How to build a big bore stroker donk
Enlighten me :lol:

Once I finally get myself another mini I would really prefer to build it around a decent (eg, ballsy) engine. It will more than likely be a "Sunday" car but the intention is to set it up for track use too.

To get things started, there are a few things i'm curious about.

*Block - anything I should look for in particular?
*Crank - what is the best to use? what can you get by with if you can't afford the good ones? :lol: is there any particular crank you need specifically because it needs to be offset ground?

I'll think of more stuff as we go along

thanks guys!!!

Damo.
Author:  kazjim [ Tue May 03, 2005 9:21 am ]
Post subject: 
ok, I've never done one... but I've read an awful lot ....

From memory you would like a 12G1505 Crank ....

if you want the biggest, baddest motor in the "A" Series world, you need to offset grind the crank a LOT and use 0.080" undersize bearings... (see Matt, i never forget!) - aparantly they are as rare as chooks choppers now, but . . .

the BLock needs to go out to 74.5mm and the bore spacing will change - you basically offset the bores 0.015 towards the ENDS of the block to maximise the space between 2 and 3 .... you also need to move two of them 0.005" towards the front of the block....

I Believe that this is Matts motor (MiniMan) and its about 1500cc !!

Need better details because most of this will be crap, but its a starting point !

J
Author:  FatMaserati [ Tue May 03, 2005 9:25 am ]
Post subject: 
Isn't 1500cc pushing it? 1430cc has very little meat between the cylinders as it is.
Author:  fuzzy-hair-man [ Tue May 03, 2005 11:32 am ]
Post subject: 
Hence the offset bore bit which moves the cylinders out towards the ends of the motor and away from the thin gap between 2 and 3. :wink:
Author:  TheMiniMan [ Tue May 03, 2005 12:41 pm ]
Post subject: 
Good on ya James,,,you`re bloody close :-)

the "Average" stroker donk is made up of a STD/STD large journal crank that has been ground offset to STD Cooper "S" size ( 1.625" from memory),,,

what we do tho, is grind em down offset to about 2 to 3 thou above that size ,,,& then balance the crank,,, & then harden the crank,,, & then grind it all true to bottom clearance size (leaves heaps of room for lots of oil & makes for a very stronk crank indeeed & that way it also makes sure that the crank is all nice & straight after all that work)

we then find a set of Cooper "S" rods, make sure they`re straight for starters,,, fit ARP rod bolts,,,& then re-size them to make them all nice & round & to size in the bearing hole :-)

easy to understand so far???

Then we offset bore the block to 73mm (& prefferably a thick flange "1100S" type or late 1275 moke or 1275LS thinck flange,,,basically any of the thick flanged "Non-side plate" blocks,,,altho there really is nothing wrong with the side plate "S" style blocks at all for road use expecially, use it if you have it, no problemo.

The 74.7mm (Triumph 2500 pistons) one that i used for my 1510cc engines has shown absolutely no drama for 8 years of my abuse until recently it has developed a problem with the centre main bearing recieving a heap of crud from the very old worn out old very 2nd hand & abused gearbox that i simply left in there way to long (naughty me really, not a fault of the engine at all, just my stupid lazyness)

The crank in that engine was infact, as James suggested , an .080 thou undersize thing that really should have gone bang years ago, but has just taken everything that i gave it for those 8 abusive years :-) & i cannot for the life of me find anymore .080" undersize big end bearings anymore,,,& trust me, i`ve hunted hi & lo world wide,,,if there is anyone out there with another new set, please ring me or e-mail me as i only have one more spare set :-( & i`d dearly love to have another set for my other 1510cc donk :-)

But,,i really wouldn`t suggest going this big, as said, there`s big problems finding the B/E bearings ,,,main problem #1,,,,& it`s a particularly suspect build for longetivity,,,yeah, i got years out of mine,,, & probably would have got lots more years out of it if i wasn`t so lazy & changed out the gearbox earlier,,,(i will simply grind the mains & build it back up again anyway, there`s nothing wrong with the big ends,,,so it will live again, that`s for sure) however, it`s not an engine you`d want to throw all your money at, "IF" it was just going to throw a leg out of bed (IE:conrod through the block or something similar)

we have found the best receipe to be an age old prooven method that my dad designed years & years & years ago (approx 1969) & that has simply not changed for all that time & it`s as stated above for crank & block being offset ground to bore of 73mm & crank to 84mm stroke (pistons are my dad`s own design & are rock solid bullet proof items & avalable from him (Jack) at ReadSpeed MiniMecca -ph-(07) 3245 2384 Chandler, Brisbane)

very carefull engine assy , paying close attention to all clearances & allowing more clearance to the lower center main cap thrusts to allow the crank to flex without pushing on the main cap ,,,& a decent cyl head set up to large inlet valves & std exhaust valve sizes with 9.5:1 or maybe up to 9.8:1 compression ratio for road engines (with re-shaped, de-shrouded & pollished chambers & running electronic ignition dist set up with the correct ign curve) ,,,we run up to 13.7:1 on race engines with this same sort of build, only fitting a huge race head & cam & SC/CR gearbox & all other race related gear tho

sorry but my fingers are now getting very sore tips on em,,,so i`ll say ta ta for now,,,any more info you need just type here & i`ll reply again soon

cheers

I would suggest that you get someone who "KNOWS" how to build performance mini engines to build it for you tho,,,lots of money to throw away if you do something a little "stupic" :-)

& please make sure you spend more money on the engine that you think you need to,,,it`s allways a good thing to spend more on the internals of the donk than spending it all on pretty valve caps & shiny tow balls & fuzzy dice hangin off the mirror ,,,if you know what i mean :-)


Oh!!!...& also please have some decent disc brakes fitted :-) :-) :-) :-) :-)
Author:  FatMaserati [ Tue May 03, 2005 12:59 pm ]
Post subject: 
Yeah, I gotta put discs on my 1430. The drums aren't even top notch, too, so weeeeeeeeeeee DEAD.
Author:  Metalfab_101 [ Tue May 03, 2005 1:03 pm ]
Post subject: 
Would Metro turbo block/crank/rods be suitable for a stroker?

I think I read somewher that they are a bit stronger than normal gear? Prolly just BS but ya never know

cheers

Damo
Author:  TK [ Tue May 03, 2005 1:21 pm ]
Post subject: 
Can't use the Turbo rods, you need 1.625 rods, ie 177,521 or 625s.
Author:  Metalfab_101 [ Tue May 03, 2005 3:00 pm ]
Post subject: 
TK wrote:
Can't use the Turbo rods, you need 1.625 rods, ie 177,521 or 625s.


OK yep I can see that. How about the crank?
Author:  TheMiniMan [ Tue May 03, 2005 6:04 pm ]
Post subject: 
Yeah, you can use the 1275 metro crank, just that it leaves little semi circular lines around the area of the crank journals that the grinder didn`t touch,,,not toooo much of a problem but i`m not one for "stress raising" lines on crank journals. i`ve used those cranks fairly recently for stroking & none have given any trouble so far, so , if you have one then use it,,,probly be the closest thing to STD?STD that you`ll find these days anyways,,,i don`t use worn or allready ground cranks for stroking as you won`t get the full stroke, you will end up with less stroke than possible with a STD/STD large crank,,,& really, you are doing it for the most that you can get "safely" right???,,,so an Un-worn STD/STD large journal is the go,,,if it`s a metro crank, then so be it.

The blocks can be a bit suspicious tho,,,lots & lots of these blocks were sleaved from the factory,,,(can`t use em for big bore then) & lots that weren`t sleaved probably should have been, for the pourosity found in them,,,YUK!!!,,,they have extra strengthening ribs in them yes,,,so they "LOOK" like they`re stronger, when in my books, i feel that they ribbed em because they knew the castings were "NOT" as good as the old ones so they`re just trying to gain some strength back...not pretty,,,so ,,,be very carefull picking a 1275 metro block for use as an offset bored big bore

& rods,,,well,,,you`re learning now,,, good to see,,,you need "S" rods & prefferably the 521s,,,, much-ous-better-ous,,, & then they have to be narrowed to the same width as the Large journal rods,,,we generally take 15thou off per side (total 30thou), sorry, forgot to mention that earlier, probly forgot to mention heaps of stuff :-)

:-)
Author:  Metalfab_101 [ Tue May 03, 2005 8:06 pm ]
Post subject: 
Thanks for typing all that Matt :shock: That all makes sense to me and has cleared things up a bit.

Then I thought, what does this do to the compression ratio? I am going to do some reading up on CRs so I might have figured this out before someone replies. Or maybe not :lol:

Damo
Author:  minitech racing [ Wed May 04, 2005 8:44 am ]
Post subject:  1430
Gday in the process of building my second race 1430 now. Hope you have plenty of money to spend as my last one cost 20 grand. If you go ahead i can do the crank wedgeing,rod lightening and have a 4 bolt main cap for sale plus a twin split weber set on longman manifold suited to a 1430.
Author:  minitech racing [ Wed May 04, 2005 8:45 am ]
Post subject:  1430
Gday in the process of building my second race 1430 now. Hope you have plenty of money to spend as my last one cost 20 grand. If you go ahead i can do the crank wedgeing,rod lightening and have a 4 bolt main cap for sale plus a twin split weber set on longman manifold suited to a 1430.
Author:  Mini Mad [ Wed May 04, 2005 11:21 am ]
Post subject: 
how much power is a $20,000 race 1430 putting out these days if you don't mind telling?
Author:  Metalfab_101 [ Wed May 04, 2005 2:51 pm ]
Post subject: 
OK OK OK, did a bit of sniffing around but couldn't really find what I wanted on the 'net with regards to compression ratio. BUT I had a bit of a think about it... yeah usually a bad move but anyway :lol: so compression ratio, is this the ratio between the swept volume of the cylinder and the volume of the head (plus head gasket thickness plus deck height plus/minus cc of the piston depending on whether it's concave, flat or convex)????

IF that is correct then I calc'd the compression ratio to INCREASE with an increase of swept volume. That being the case, the CR should also increase when you overbore an engine, as this is also increasing the swept volume.

So if all that dribble is correct, what do you do about the CR when you stroke out the engine? If you're doing a larger bore at the same time then that will make the compression ratio even higher. Based on my thoughts you should be able to grind out the head more (larger cc chamber will drop the CR) or go for dished pistons (or flats if you had high CR convex face pistons previous).

But that's enough BS from me, what do you guys actually do with the CR when you bore & stroke???

:)

Damo
Page 1 of 4 All times are UTC + 10 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/