piston masses

[jbuckl]

recent findings about pistons:-

original piston std	473.9	grams
original pin	125.2	grams
original total	599.1	grams
county piston +0.040	437.6	grams
county pin	125.2	grams
county total	562.8	grams
original heavier than county by	36.3	grams

Only one piston was weighed.
So, mass may need removing from counter weights of the crankshaft to adjust balance if +0.040" county pistons are being used .
The approx. drill size & depth can be calculated
Since the crank is balanced using ½ the reciprocating mass & pistons are all reciprocating mass, for each piston this means a drilling to remove 18g per piston needs to be made.
Each drilling can be ½ inch or 12.5 mm dia. & 23mm deep. (this will remove approx. 18grams of metal @ std. drill point angle. (@Density 7.2000000e-03 GRAM / MM^3)
Drillings could be combined to suit the counter weights.
The ‘proper way’ would be to construct ‘bob’- weights for each journal though & dynamically balance.

welcome any comments regarding the above ~ after its been read.

[jusm]

Well I concur on the weight! Plus or minus a bit...
5387DBE5-DAF8-49F1-9C45-34610F827FED.jpg

[Motorsport Micky]

"Each drilling can be ½ inch or 12.5 mm dia. & 23mm deep. (this will remove approx. 18grams of metal @ std. drill point angle. (@Density 7.2000000e-03 GRAM / MM^3)
Drillings could be combined to suit the counter weights.
The ‘proper way’ would be to construct ‘bob’- weights for each journal though & dynamically balance.

welcome any comments regarding the above."

I agree with your statistical figuring (within a nats nadger) but please tell me you are a professional balancing shop or have access to one and you are not going to start drilling your crank yourself !

Micky

[jbuckl]

Not my project.
I'm not drilling anything.
I'm not a balance shop.
It's information which may be useful. (there was information posted here before, saying that county pistons were heavier than hepolite !!!)
I should have included 'after its been read' Apologies for that.

If someone chooses to improve the balance when using dis-similar mass pistons, it is possible, without using dynamic balancing & bob-weights with the information provided.

If anyone knows of a reputable dynamic balancing company who actually already has adjustable bob-weights for a stag crankshaft, that would be helpful.

There are other dynamic balance methods available, but the costs to set up are normally out of reach for our level of expense, especially for a one off.

"Each drilling can be ½ inch or 12.5 mm dia. & 23mm deep. (this will remove approx. 18grams of metal @ std. drill point angle. (@Density 7.2000000e-03 GRAM / MM^3)
Drillings could be combined to suit the counter weights.
The ‘proper way’ would be to construct ‘bob’- weights for each journal though & dynamically balance.

welcome any comments regarding the above."

I agree with your statistical figuring (within a nats nadger) but please tell me you are a professional balancing shop or have access to one and you are not going to start drilling your crank yourself !

Micky

[Mark Serj]

Just noticed this thread. I have just fitted +20 county's and they were heavier than the std size pistons I removed, & IIRC the County pins were lighter. all my records are on another PC so I'll check Monday & let you know.

[Mark Serj]

Heavy original pistons?

My figures below :

Std mk1 pistons weight ave. 424.5g (they have FK71 stamped on them - poss to match the original bore grade?)
Std ring set 40g
ave. pin 126g
ave std piston assy weight 590.5g

Lowest +20 county piston 432g
County pins all 138g
+20 rings 43g
Total +20 county piston assy weight 613g

I equalized all my piston weights to 432g, & equalised the rod dimn from pin to top across all 8

Taking into account the loss of 1.3g on the reground crank & weight removed from small ends, I removed in the region of 11g from the rod & cap big ends (approx. 1/2 the reciprocating mass weight difference) bringing the whole set to exactly the same weight.

Will probably make naff all difference but its fun to do best you can with what you have!

Better go, Dad taxi time!!!

[Mark Serj]

PS Inspired by the Flying Farmers doings BTW!

[flying farmer]

Had a thought about the weight of your original pistons. Were they by any chance mk2 domed sort?
If they were it is a possibility they were the much heavier JP pistons from Australia.

I got caught out by this when I rebuilt my P reg Stags engine. They use different piston rings and I found the County set I had bought didn't fit. The JP pistons have both the compression rings the same thickness, the top one being the same as the county middle ring rather than being thinner.

[Mark Serj]

I wondered the same