10 replies to this topic

[Ciccio-kof]

Hello everyone, as stupid as it sounds, has anybody ever came across a factory crankshaft where pistons 2 and 3 have a different stroke than 1 and 4?? We are talking about 2/10 of a millimeter, but I'm actually going crazy with it.

Measured all the pistons, they all have the same compression height. Also the con rods are all the same, when I thight them at the crankshaft I have 2/10 of a mm difference when at TDC.

Am I going crazy or what??

[imack]

0.2mm at tdc or 0.2mm different between bdc and tdc?
Poorly ground crank or maybe a stack up of factory tolerances.

[Ciccio-kof]

0.2mm between the top of the block and the piston at TDC. Crankshaft is factory std, never been machined or anything...what is your suggestion? At this point? I would have to calculate CR in order to have it consistent between the 4 cylinders...

[Spider]

Yes, most as they come from the factory are off in either stroke or angle or both, only by a thou or 2. Just looking at a drawing (not a terribly clear copy) for a 998 Crank, the tolerances seem to be generally around +/-0.00035"  (3-1/2 tenths of a thousandth), but I've found loads well outside that ! Having them re-ground on the correct type of grinder, that holds the crank between indexable centres, can correct this easily, but if the shop has one of those Prince Grinders or similar, it will only resize the journals, but not correct where they are. I really don't like this style of grinder as it only follows where the journal is, so it could be off from manufacture, wear or both. Because of this, I have little respect for the shops who have them.

 

When I blue print engines, is is definitely one Item I have done.

[Ciccio-kof]

Well I wouldn't mind a thou or two, but here we're talking about double the size of it. 

 

I think it'll be really hard to find one of those machine shop here were I'm from, as I am building a moderately hot street engine I think I would just do CR independently for cylinder 1, 4 and 2, 3 .

 

Don't think it is going to make that much of a difference performance and durability wise, what do you think? 

 

Really curious to know what other people would have done as well     

 

 

Yes, most as they come from the factory are off in either stroke or angle or both, only by a thou or 2. Just looking at a drawing (not a terribly clear copy) for a 998 Crank, the tolerances seem to be generally around +/-0.00035"  (3-1/2 tenths of a thousandth), but I've found loads well outside that ! Having them re-ground on the correct type of grinder, that holds the crank between indexable centres, can correct this easily, but if the shop has one of those Prince Grinders or similar, it will only resize the journals, but not correct where they are. I really don't like this style of grinder as it only follows where the journal is, so it could be off from manufacture, wear or both. Because of this, I have little respect for the shops who have them.

 

When I blue print engines, is is definitely one Item I have done.

[Shooter63]

Unfortunately most shops have the Prince type, I have exactly the same problem no1 is flush no2&no3 are 4thou down and no4 1 thou above, luckily it plays into my plan a bit anyway, I'll be running quite a stout compression ratio as I'm trying to build as much torque as possible and evidently A series engines detonate on no2 first.

Shooter

[DeadSquare]

Old Harry Ford

took some rubber and a can,

nailed it to a board

and the darned thing ran.

 

I know that this is not what you want to hear, but just put it all back together.

It is not a racing car, where you are striving for the last 1/2 BHP., and I wouldn't worry too much about the compression ratio.

In 1960, my mother bought me a Morris 1000 Traveller, and under the spare wheel was a compression tester.  Not a good sign and one cylinder was low, but with a supercharger, it would almost do a ton.

I have built quite a lot of engines over the years, and have never found all four cylinders to give exactly the same reading.

[Spider]

At the end of the day building it, you can only work with what you have available to you, be a crank that's off or available services to sort it out. As the other guys have said, it's not going to make big differences in it running, especially if it's a street car - usually.

I've generally tried to strive for getting the squish area as thin as possible and that has sometimes meant bringing the piston crowns above deck height. That's when details like this become critical. It also makes it difficult or even pointless going to the nth degree timing a cam in of the crank indexing is out.

[Cooperman]

Not only is it normal for the stroke dimensions to vary slightly, the angular accuracy is often slightly off.
With road engines it's not an issue, but for race engines it's common to have the crank 'stroked and indexed' to remove these tolerances.

[Ciccio-kof]

That's a bit reassuring, I mean as I mentioned on a different post I'm doing this job with my dad, he scared me a bit by saying that is not very common for crank to have that much tolerances, even factory once, but I have to say he is not use to A series engines, I will work some math out since the engine is stripped again and do my best, will keep you updated with some pics and a video of the running engine hopefully by the end of the month. Thanks to all of you for advices :)

[stuart bowes]

Old Harry Ford

took some rubber and a can,

nailed it to a board

and the darned thing ran.

 

 

I heard it as..

  a bit of old tin, some rubber and a board

  nail 'em all together and you've got a ford

 

(I do love a ford though so it's all just banter)

 

looking forward to all this interesting engine stuff when I finally get the bodywork out the way

Edited by stuart bowes, 13 July 2023 - 10:46 AM.