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1275 Block Deck Height And Pistons Selection


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#1 Inno

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Posted 06 March 2019 - 09:28 AM

I think I checked all topics I could find but didn't find out is there a possibility to measure block deck height if I only have a block.

 

I understood that to be sure what components I need, I need to measure deck height with present pistons and chamber volume and decide what will be desired CR and then decide about new pistons (type, dish, crown dimensions).

 

But, what if I do not have any pistons right now?

 

I have A 1275 block and no clue what was happening last 30 years with it.

 

I bought Innocenti piston rods and I guess they are of the some standard length. Also standard A 1275 crankshaft that was in that block. I do not have any pistons and no bearings at the moment for any dummy build....

 

So is there any way to measure deck height from some block reference and to understand how much deck meat I may have and how much deck height I can expect with specific piston type on innocenti rods (Hepolit 21253 as an example)?

 

---like deck to block bottom height or any other reference?



#2 Retroman

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Posted 06 March 2019 - 09:42 AM

The only way to sort the measurements you need to know is a dry build

 

You need bearings and pistons

 

Is it just a straight forward rebuild or are you uprating the engine ?  cam / head / pistons ?

 

The fuel and cam dictate what the compression should be

 

This might help when it comes to number crunching :

 

https://jscalc.io/ca...RNEglVkKLQir11T



#3 Inno

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Posted 06 March 2019 - 10:25 AM

The only way to sort the measurements you need to know is a dry build

 

You need bearings and pistons

 

Is it just a straight forward rebuild or are you uprating the engine ?  cam / head / pistons ?

 

The fuel and cam dictate what the compression should be

 

This might help when it comes to number crunching :

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

thanks....so it means block tolerances or differences are just to big to do any reliable block measurement...

 

...It will be an engine upgrade that is why I got Inno rods.

 

...saw the calculator in some of yours previous posts..thanks!

 

 

So it means I need to get any dummy pistons and some old but suitable bearings and mount pistons to rods and put all together like that to measure deck height?

 

Or check crank shaft and polish or regrind as needed, lighten it and balance and do the same with rods and buy proper new bearings and then use that for dummy built (lower part finished), but only mount any available pistons to rods for dummy deck height measurement and then decide what kind of new pistons I can buy?

 

Is it ok to use new bearings for a few dummy builds?

 

 

 

 



#4 Retroman

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Posted 06 March 2019 - 12:22 PM


 

 

So it means I need to get any dummy pistons and some old but suitable bearings and mount pistons to rods and put all together like that to measure deck height?

 


 

You can do it that way, but the best and most accurate way is use the parts you are going to use for your build.

There can be a few thou difference.

 

You can measure the pin height of a dummy piston and deck height of the same in a dry build. Then compare the pin height and calculate the difference

 

I use a slightly reduced daimeter gudgeon pin for dry builds as the interference fit is a pain.

 

 

Or check crank shaft and polish or regrind as needed, lighten it and balance and do the same with rods and buy proper new bearings and then use that for dummy built (lower part finished), but only mount any available pistons to rods for dummy deck height measurement and then decide what kind of new pistons I can buy?

 

Get the crank checked by the machine shop, even new cranks benifit from a polish.

 

As above you can dry build with other parts and calculate any difference. For your purposes it will be accurate enough.

 

 


Is it ok to use new bearings for a few dummy builds?

 

 

You can use new bearing for dummy builds, but everything has to be spotlessly clean making sure you only use the same bearings on the same journals too. So each shell needs marking as big end bearings [+ early mains] are the same top and bottom.

 

The oil gallery plugs and every fitting needs to come off / out the block before machining then a very thourough

 

cleaning of all oil galleries, including those in the crank and head before any build [and everything else]

 

A wash tank, pipe cleaners, a long welding wire, brake cleaner and an airline all help....it can never be too clean



#5 Inno

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Posted 06 March 2019 - 12:56 PM

thanks a lot....so if I really would like to be precise and sure I have exact clearances I shall do everything on the crankshaft and lower part, buy new bearings and then do the measurement with dummy pistons in order to be able to decide which pistons to use....and how much to skim the deck (or pistons) if needed to achieve wanted CR....



#6 Moke Spider

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Posted 06 March 2019 - 06:18 PM

All great advice from Retroman.

 

The only thing I'll add is when doing a dry build for this purpose, you don't need to fit the Piston Rings.

 

A modification to consider too, is to make the Press Fit Pins a Floating Pin. The Machine Shop can Hone out the Little Ends on the Rods to give 0.0008" clearance here. This really speeds up Dry Building, especially if you need to remove the Pistons from the Rods a few times.

 

To retain the Pin, you just fit Teflon or Aluminum Buttons.

 

This is a mod I routinely do when building performance engines.

 

It also removes forever the risk of the Pin going for a walk on it's own,,,,



#7 Cooperman

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Posted 06 March 2019 - 06:44 PM

I have a set of linished down pins which are about 0.0005" smaller diameter.
These make the trial build very easy.
And yes, remove the rings from the new pistons before doing the trial build, but do it carefully. It is so easy to snap a ring and you cannot buy them individually!

#8 justminis

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Posted 06 March 2019 - 09:58 PM

 

 

A modification to consider too, is to make the Press Fit Pins a Floating Pin. The Machine Shop can Hone out the Little Ends on the Rods to give 0.0008" clearance here. This really speeds up Dry Building, especially if you need to remove the Pistons from the Rods a few times.

 

To retain the Pin, you just fit Teflon or Aluminum Buttons.

 

This is a mod I routinely do when building performance engines.

 

It also removes forever the risk of the Pin going for a walk on it's own,,,,

 

I've done the floating pin conversion in the past and the teflon buttons work well with full skirt pistons, where the gudgeon pin length is close to the diameter of the bore.  Ever done it with slipper type pistons, where the pin is much shorter in length than the bore dia.?  Building an engine atm and considering this mod but the pistons are the slipper type.  Prefer floating pins over press fit every day of the week.


Edited by justminis, 06 March 2019 - 09:59 PM.


#9 Retroman

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Posted 06 March 2019 - 10:37 PM



thanks a lot....so if I really would like to be precise and sure I have exact clearances I shall do everything on the crankshaft and lower part, buy new bearings and then do the measurement with dummy pistons in order to be able to decide which pistons to use....and how much to skim the deck (or pistons) if needed to achieve wanted CR....

 

To be really precise you need the new bearings on a refreshed crankshaft.  It should be checked with plastiguage on each journal to make sure the bearing clearances are good. [Yes machine shops get it wrong] The crank end float should aslo be checked [a dial guage is best] and set at what matches your application, the bottom end is then sorted.

 

Decide which pistons first....they depend on what the engine will be used for and budget

 

Then dry build / measure / skim [block, head and or pistons] to get the CR needed...

 

Which again depends on existing condition, engine use, fuel and camshaft.

 

Not sure what grade of fuel is available in Zagreb ?



#10 Moke Spider

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Posted 07 March 2019 - 03:51 AM

 

I've done the floating pin conversion in the past and the teflon buttons work well with full skirt pistons, where the gudgeon pin length is close to the diameter of the bore.  Ever done it with slipper type pistons, where the pin is much shorter in length than the bore dia.?  Building an engine atm and considering this mod but the pistons are the slipper type.  Prefer floating pins over press fit every day of the week.

 

 

Yes, I have done them with AE Pistons (when I was still using them) and Hypertecs.

 

I don't make them a tight fit in the Pin and I also drill an off set vent in them. If the vent were done on centre, it would get blocked off on the wall of the bore.

 

If you are concerned that there's too much hanging out, make them in Aluminum.



#11 johnv

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Posted 07 March 2019 - 09:01 AM

serious question from someone recently started dabbling in engine building .. If a good trusted engineering shop has checked the block (ie doesn't need line boring of the crank journals) , polished and/or reground crank as necessary, why do you need to go any further than fit the crank and check it all spins freely, then check the end float? 



#12 Fast Ivan

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Posted 07 March 2019 - 12:08 PM



serious question from someone recently started dabbling in engine building .. If a good trusted engineering shop has checked the block (ie doesn't need line boring of the crank journals) , polished and/or reground crank as necessary, why do you need to go any further than fit the crank and check it all spins freely, then check the end float? 

 

because of this

 



make sure the bearing clearances are good. [Yes machine shops get it wrong] 

 



#13 johnv

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Posted 07 March 2019 - 12:12 PM

 



serious question from someone recently started dabbling in engine building .. If a good trusted engineering shop has checked the block (ie doesn't need line boring of the crank journals) , polished and/or reground crank as necessary, why do you need to go any further than fit the crank and check it all spins freely, then check the end float? 

 

because of this

 



make sure the bearing clearances are good. [Yes machine shops get it wrong] 

 

 

 and what if they're not? back to the machine shop?



#14 Fast Ivan

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Posted 07 March 2019 - 12:23 PM

yep

 

assuming that what you've bought off the shelf or had machined to your specifications is correct, without checking, is not a good strategy

 

I've returned a crank that was machined by a very reputable source after I measured the journals, no problems at all and a solution was very prompt, the point is I found the problem before bolting it all together



#15 nicklouse

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Posted 07 March 2019 - 12:24 PM

 

 



serious question from someone recently started dabbling in engine building .. If a good trusted engineering shop has checked the block (ie doesn't need line boring of the crank journals) , polished and/or reground crank as necessary, why do you need to go any further than fit the crank and check it all spins freely, then check the end float? 

 

because of this

 



make sure the bearing clearances are good. [Yes machine shops get it wrong] 

 

 

 and what if they're not? back to the machine shop?

 

yep one reason I let the shop do the build. tell them what is needed. collect and then install. simples.






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