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Old 15-03-2019   #1
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Benefits of Bis Crank

I'm gathering parts for a 700cc engine build and will need to select a crank sometime in the near future. I have been advised by several people that a bis crank is the best option for a tuned engine can anyone explain why?

  • If using bis pistons, the bis crank makes sense because the weight is matched. (heavier pistons = heavier crank counterweight)
  • The centre of mass is closer to the axis but this is proportional to the mass used. E.g. 500g mass at 2" from centre will have the same moment as 1Kg mass at 1" from centre. so this seems like a moot point.
  • The profile design means that the lowest point of the crank is very narrow. I believe this is for barrel clearance on the bis but the effect could be similar to knife edging, although I haven't read anything conclusive about knife edging since there is very little oil in the bottom of the sump to cut through while the engine is running. SO this may also be a moot point.
Happy to hear any other thoughts on this that might clear it up!
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Old 16-03-2019   #2
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Re: Benefits of Bis Crank

Quote Originally Posted by MOD500 View Post
I'm gathering parts for a 700cc engine build and will need to select a crank sometime in the near future. I have been advised by several people that a bis crank is the best option for a tuned engine can anyone explain why?

  • If using bis pistons, the bis crank makes sense because the weight is matched. (heavier pistons = heavier crank counterweight)
  • The centre of mass is closer to the axis but this is proportional to the mass used. E.g. 500g mass at 2" from centre will have the same moment as 1Kg mass at 1" from centre. so this seems like a moot point.
  • The profile design means that the lowest point of the crank is very narrow. I believe this is for barrel clearance on the bis but the effect could be similar to knife edging, although I haven't read anything conclusive about knife edging since there is very little oil in the bottom of the sump to cut through while the engine is running. SO this may also be a moot point.
Happy to hear any other thoughts on this that might clear it up!
Hi Mod, My choice of the BIS crank was based on the mass being closer to the centreline and thus theoretically able to tolerate higher revs before 'whipping' (flexing) sets in (without balancing). Higher quality bearings and bolting a 4ltr alloy sump direct (only silicon gasket) to the crankcase helps to stiffen the assembly.
I am using standard BIS pistons and Alloy 'H' section rods that have been matched and balanced, The counterweight of the crank has been Reduced in weight as part of the balance, so it is dependant on the piston/rod/crank matching not just the piston and crank weight. The BIS engine uses wet liners and I too believe that the profile of the counterweight is narrow to suit this arrangement. I don't think that knife edging would give any appreciable gain as the rotating mass in the sump does not have a great stroke and thus will not plough the oil deep enough to make it matter, but it may be possible for a light weight racing application to gain a small percentage if used, I don't know if European racers do this?
Ian.
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Old 17-03-2019   #3
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Re: Benefits of Bis Crank

I am in the process as time but most importantly, money allows, of building up a '695' engine. When discussing with an expert this very question, 2 reasons came up as good reasons for using a BIS crank---(1) the 126 BIS did not have a good reliability record, so most of the scrapped cars/engines had relatively low mileage on them and, (2) the narrowness of the BIS crankshaft balance weight would probably be of benefit when fitting big barrels/pistons. I am told that the BIS crank is still'safe' up to 6,000rpm, with an occasional burst to 6,500rpm. If one is going to run ANY 500/126 crank regularly over 6,000rpm I would recommend that the crank-shaft core plugs are 'TIG-spotted' to prevent them being blown out, with a subsequent severe loss of oil pressure!
Personally, I can't see the point of going over 695cc (OK, I will concede 700cc). Abarth had 2 very good reasons for not going above this capacity---(a) you cannot safely bore out the crankcase of the 110 engine any bigger than that required for a 652cc capacity and, (b) you cannot safely go above a 76mm stroke. The availability of the 126 crankcase has changed this limitation, but much above 700cc tends to produce an engine that runs hot and coarsely.
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Old 17-03-2019   #4
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Re: Benefits of Bis Crank

Quote Originally Posted by the hobbler View Post
Personally, I can't see the point of going over 695cc (OK, I will concede 700cc). Abarth had 2 very good reasons for not going above this capacity---(a) you cannot safely bore out the crankcase of the 110 engine any bigger than that required for a 652cc capacity and, (b) you cannot safely go above a 76mm stroke. The availability of the 126 crankcase has changed this limitation, but much above 700cc tends to produce an engine that runs hot and coarsely.
Possibly Camilo Alquati was a little more frivolous than Abarth as he seemed to go for the 795cc conversion in the 1980's . When I recommissioned my engine I was very surprised to discover that it was based on an original 594cc crankcase so the land to locate and seal the barrels was almost non existent but the seal was achieved by making the barrels a press fit into the crankcase. The crankshaft is standard and clears the pistons OK.
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Old 17-03-2019   #5
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Re: Benefits of Bis Crank

Greetings, It's always puzzled me when considering modifications to the basic 500/126 engine that, of all the available mod kit's none (to my knowledge) even mention the crankshaft assembly, let alone supply parts to upgrade it. It's all for the top end in various sizes and material choices ???
I would have thought that even a little consideration would go a long way in making sure everything stays together at high revs. Only third party individuals who have gone down this road mention the bottom end at all I'm not sure what rev limit I need to set with everything balanced, but the German builder IL MOTORE builds and sells a monster 900cc engine that gives 70 BHP at 7000rpm How do they achieve that without it coming apart at the seams?????
Ian.
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Last edited by Bleeding Knuckles; 17-03-2019 at 15:53.
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Old 17-03-2019   #6
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Re: Benefits of Bis Crank

Quote Originally Posted by Bleeding Knuckles View Post
IL MOTORE builds and sells a monster 900cc engine that gives 70 BHP at 7000rpm How do they achieve that without it coming apart at the seams?????
Ian.
A custom crank must be prohibitively expensive unless you are with a re=acing team in which case you would probably keep that design to yourself to maintain a competitive advantage. I've heard of the sump being used as a block stiffener, I've also seen reinforcement plates fitted between the block and the sump.


Lightening the pistons and con rods allows more crank weight to be shed which will reduce the crank loads at higher revs. I'm trying to find few books on tuning single cylinder engines at the moment in the hope that it will shed a bit of light on the situation.

I'm happy to buy things if needed, but I much prefer to understand why first!
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Old 18-03-2019   #7
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Re: Benefits of Bis Crank

Can anyone say in which respects the design of the BIS crankshaft might be influenced by the fact that it is used in a horizontal engine?
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Old 18-03-2019   #8
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Re: Benefits of Bis Crank

Quote Originally Posted by fiat500 View Post
Can anyone say in which respects the design of the BIS crankshaft might be influenced by the fact that it is used in a horizontal engine?


Good question.
I don't know enough about dynamic balancing to know the answer. I would hazard a guess that the since the Bis engine is horizontal, the crank weight does not have to overcome piston gravity, just the mass of the piston. This will make the piston+conrod assembly lighter and should result in the crank being lighter. The pistons are slightly heavier due to the larger diameter.


The oilway through the crank may be different, but I've never investigated. Has anyone weighed a standard Bis crank and compared to a standard Aircooled Crank?


There you go, I've answered a question with a question, most unhelpful!
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Old 18-03-2019   #9
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Re: Benefits of Bis Crank

Quote Originally Posted by MOD500 View Post
Good question.
I don't know enough about dynamic balancing to know the answer. I would hazard a guess that the since the Bis engine is horizontal, the crank weight does not have to overcome piston gravity, just the mass of the piston. This will make the piston+conrod assembly lighter and should result in the crank being lighter. The pistons are slightly heavier due to the larger diameter.


The oilway through the crank may be different, but I've never investigated. Has anyone weighed a standard Bis crank and compared to a standard Aircooled Crank?


There you go, I've answered a question with a question, most unhelpful!
I see I've previously asked the same question and got agood answer.
https://www.fiatforum.com/500-classi...ml#post4367801
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Old 18-03-2019   #10
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Re: Benefits of Bis Crank

Quote Originally Posted by fiat500 View Post
I see I've previously asked the same question and got agood answer.
https://www.fiatforum.com/500-classi...ml#post4367801

Looking back through the threads you linked to brings up an interesting point with the Giaridiniera where a counterweight is added to the flywheel. Assuming this is to counter the issue that gravity has been rotated by 90 degrees it is fair to assume that the bis has the work carried out on the crank rather than picking up the error at the flywheel.
I have read an article that covered the position the counterweight starts and ends which is as critical in it's position as the lobes of the cam shaft. Manufacturers start with an overweight crank and remove weight. The location they remove the weight from could be adjusted to suit a horizontally in line engine or a vertically in line engine.
If you take a BIS or an A/C crank to be dynamically balanced then the position of the crank mass doesn't really matter, it will be adjusted to suit your engine.
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Old 18-03-2019   #11
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Re: Benefits of Bis Crank

Quote Originally Posted by MOD500 View Post
The oilway through the crank may be different, but I've never investigated. Has anyone weighed a standard Bis crank and compared to a standard Aircooled Crank?
There you go, I've answered a question with a question, most unhelpful
I had never thought of that and I kind of assumed that the two crankshafts would have been pretty much the same weight. So having just moved a Bis and a standard crankshaft at the weekend I had them to hand but had to use my bathroom scales so not the most accurate but to my surprise the standard crank was showing 4.8kgs and the Bis crank 5.2kgs.
I have been running a Bis crank in my tuned 126 engine and it seems to perform quite well. I have 3 other spare Bis crankshafts, one brand new with bearing set, one reground with bearing set and one used standard size. Also got some standard reground crankshafts with complete bearing sets.
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Old 18-03-2019   #12
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Re: Benefits of Bis Crank

Greetings, Given Dave's crankshaft weight figures I'm now not surprised that my BIS crank has been reduced in weight, even though I'm using standard pistons, but Alloy rods. I can confirm that the oil gallery in the BIS crank is not different - it does take a lot of cleaning out though My standard flywheel has been reduced in weight to 4.56kg and had very little material removed to balance. but it all has to go back together a particular way I will enquire of my balancing workshop re the difference (in balance terms) between horizontal and vertical engines and see what they say.
Ian.
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Old 19-03-2019   #13
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Re: Benefits of Bis Crank

It seems that the air-cooled crankshaft weight/design stayed the same for the horizontal Giardiniera engine and through increases in piston weight through the 594cc and 652cc engine version's. That being the case, it could be interpreted that the important thing to achieve is the intrinsic balance of the crankshaft itself, presumably done at the factory, rather than the counterbalance weight being matched to the pistons.
The forces exerted on the crankshaft are made up of much more than the weight of the pistons; the motion is not simply in two opposing directions but is rotary and involves centrifugal force.
I think that the fact that @Toshi 975 has achieved a vibration-free engine without having to seek complex technical input may be an indication of this.
It seems possible to me that removing or adding weight to the crankshaft might actually cause instability and render the component unusable in any other configuration.
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Last edited by fiat500; 19-03-2019 at 07:18.
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Old 19-03-2019   #14
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Re: Benefits of Bis Crank

I Agree that the mass of the pistons would make up a very small amount of the balancing issue, I was trying to think what would be different between the A/C and Bis crank setup.


I have heard a few very smooth running engines that were factory standard. Fiat manufactured to a tolerance that was large by todays standards. Some cranks were over balanced in places, some were under balanced in places and some were spot on. (By luck more than judgement!)
Maybe the newer Bis crank, as a starting point, is already manufactured to a more modern tighter tolerance, so it's better to begin with.
If that is the case.(No Idea how to confirm that.) Then the bis crank would be a better choice if you don't want to have the engine balanced.
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Old 19-03-2019   #15
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Re: Benefits of Bis Crank

Greetings, I think that the overall balance of a 'standard' engine is down to the manufacturing tolerance of all of the component parts. Fiat will probably have accepted that the engine assembly will vibrate and have an out-of-balance factor that is within tolerance. You may get a smooth engine with a certain set of components and you may get another that is apparently rougher in operation. The level of cost outlay for perfectly matched parts is prohibitive if the maker is going to make any profit at all. I have not had the luxury of getting my hands on a complete BIS engine to remove the Crank, Pistons and Rods from, so I accept that my assembly of parts from other places has meant that a measure of balancing is required to be sure of a smooth operation. I also appreciate that balancing an item for one engine will not immediately make the same item applicable elsewhere. You accept that your own particular build is within your own tolerance or not. I will just add that My own BIS crankshaft came from a late 80's engine and as standard had an out-of-balance level of 3290 gm/mm - after balancing the level dropped to 20.1 gm/mm .
Ian.
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