Engine balancing (1300)

B0b

B0b

Daily Driver
got a bunch of parts today, including 87mm pistons.
They look OK but the piston and pin weighs 60 grams more than the originals.
Some of it is just a different casting, but some is also just due to the diameter being larger, I guess.
I had originally planned on working the pistons and pins to match the weight of the originals, but there is no way to remove 60 grams.
As I recall, balancing an inline 4 does not use bob weights on the crank, because the weight of the rods and pistons cancel out, as long as they are weight matched. If I am correct, then the additional mass of the pistons wouldn't affect balance.

Any thoughts?
 
That is going off the assumptions I read and used as well. I matched my rods, pins, and pistons as best I could and then had the crank, and the flywheel balanced separately. I sure hope that's the right way to go about it.
 
thanks for the link.
I actually took a class on engine design when at mech eng school.
Problem is that The theory of how to approach balance doesn't answer the simple question:
Does changing the piston weight affect the balance of an inline $?

As per the post above, after the original designer of the crank decides how much of the various forces to balance out, then the crank is balanced without bob weights, as you would use on a Vee type engine. That implies that the piston and rod weight are not part of the balance.

BTW, does anyone have a 87mm piston they could weigh? just curious how the pistons I bought compare.
Mine weigh 580 grams with the pin but without the rings and the pin keepers.
The original 86mm stock ones weigh 520.
 
Bob,

You are correct. An inline 4 uses 180 degree crank throws and therefore are naturally balanced. Changing weight of the piston/rod assemblies (given that they are all the same weight) doesn't affect the natural balance.

I have built numerous performance and racing engines. Most of these using lighter, aftermarket, pistons and rods. The reduction in reciprocating weight/mass is a performance improvement itself.

The larger diameter of the 1300 87mm pistons will cause them to weigh more than OE. In addition, aftermarket piston designs will typically result in weight difference from the OE as well. All typical.

I should note that the SOHC engine, and the 1300 in particular, benefit from a balanced rotating assembly. Balancing the crank, rods and pistons is noticeable on a well tuned engine.

Also, when balancing the rods, building a jig that permits you to weigh only the big end and balance it separately, is best practice.
 
I think you know that it's more important to have all the pistons weigh the same than it is to match the weight of the factory pistons. But as lighter is better you might want to consider the weight of the pins. Are the pins in the new pistons heavier than the stock pins? If so, can you swap pins out?
You can really save weight by going to a lightweight pin.
 
Bob, I'm curious how the new pistons compare in weight from one another (i.e. the differences between each of the new pistons)?
 
Some weights and measures: (grams and mm)
piston pin dia @ 27.5mm from bottom of skirt
  1. 444.5 133.5 86.990
  2. 445.3 133.6 86.985
  3. 444.1 133.6 86.980
  4. 445.1 133.5 86.985
Stock 389.5 130.9
Added: each valve pocket in the new piston has a volume of 1cc.

The variance of the new pistons is surprisingly small, and easy to make equal.
The diameter measurements show a bit of variation but measuring down to +/- 5 microns is a bit iffy, so I'd call them identical.
The vendor said the compression height is 34.9; I measured 34.84, so that dimension is good.
The tops are completely flat, with shallow valve pockets, about 8mm deep.
The original piston have a deck height of -0.7mm, so the new ones will be about - 0.10mm.
The machined surfaces have a decent finish, and the fit of the pins is good.
The casting is a bit rough on the inside.
Here are a some photos:
2019-11-14_00001.jpg 2019-11-14_00002.jpg 2019-11-14_00003.jpg 2019-11-14_00004.jpg
 
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Regarding lighter options, I could use the old pins to save 3 grams or so, but otherwise I think i'll just live with the heavier pistons.
After all , this is a really mild engine - a European 4331512 cam and a bit more compression.
Based on my measured combustion chamber volume of 30cc, and assuming a 1.4mm gasket, and using the measured deck height, I calculate a compression ratio of 8.1 in the original form.
With the new, taller pistons, and the new 87mm bore, I calculate 8.87.
I need to clean up the head and block, so say 0.25mm off each, taking the CR to 9.3. This would result in a deck height of + 0.15mm.
Question is whether or not to take a bit more off the head and/or block, then unshroud the valves a bit to get the CR back to around 9.3.
Possibly a bunch of work for little gain, given the state of tune?
The gasket I have is 1.6mm thick; is the assumption of 1.4 compressed reasonable?
How much positive deck height is safe? or is it better to just clean up the block and do any removal from the head? maybe a total combined removal of 1mm?

any comments will be appreciated.
 
Ideally, you want the piston to head clearance between .75 and 1 mm. This improves detonation resistance. In order to get there, you must know the true compressed thickness of the head gasket and the true deck height of the new pistons when installed. If you want it on the tighter end, you need to preassemble the engine with the gasket to be used and carefully measure everything before having the block cut.
 
Just got the balanced flywheel and crankshaft back. The crank had material removed at the center and the flywheel end:
upload_2019-11-23_14-53-50.png


Not much so who knows whether it was worth it.
I had the flywheel lightened a bit, so it required rebalancing.

upload_2019-11-23_14-44-32.png
 

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Bob, wich manifacture are your new pistons? will be this motor EFI or carbed? are you planning any work on the cylinder head?
 
The pistons are 87mm "pop-up" from Vick.
The engine is a 1300, carburated, with only a couple of minor changes planned.
That includes some work on the cylinder head, after I get the block back from the machinist.
More discussion is in my Compression ratio thread.
 
B0b said:
The pistons are 87mm "pop-up" from Vick.
The engine is a 1300, carburated, with only a couple of minor changes planned.
That includes some work on the cylinder head, after I get the block back from the machinist.
More discussion is in my Compression ratio thread.
Click to expand...
Did you take it to Carl at TDC Bob?
 
Block is at TDC for boring and cleaning up the deck.
Flywheel and crank to Custom Engine Rebuilding in Ottawa.
 
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