Plastigage

[Waldi]

There is a first time for everything. In this post I want to share my first experience with plastigage, the "plastic fishing wire" which can help to determine the clearance in a bearing.

Plastigage comes in different sizes, for different ranges of clearances.

I had my crankshaft ground (-0.010"/ -0,25 mm) since the journals were all slightly below the minimum allowed diameter.

First I measured the bores of the block bearing housings (without shells installed), to verify if they were correct size and "round" (the WSM provides the required dimension). I did the same for the conrods.

After these dimensions appeared ok, the company who did the grinding determined the required size for the journals.

They have the experience, I just asked for a 0,03-0,04 mm (1,2-1,6 thou) final clearance.

On return, I measured the journal diameters with a micrometer.

I then installed the new bearing shells, and measured the inside diameter with a bore gage (which is calibrated on the micrometer that I used for the journals).

Now I could calculate the diametrical clearance. All were as I hoped, I was impressed: 0,03-0,04 mm (1,2-1,6 thou).

Since I had the plastigage filaments, I was able to do a double check, first of all "to be sure", but also because I was curious to see if the plastigage readings would match the instrument measurements.

The result was satisfying, all readings were within 0,01 mm (0,4 thou) the same!

I used a Mitutoyo micrometer, and a Dasqua bore gage.

 

 

 

 

 

[RogerH]

Hi Waldi,

I have never used the PlastiGage. You have made it quite clear. Thanks.

 

Roger

[AlanG]

It's use was demonstrated at the TRGB engine building seminar the other weekend. Very impressive.

Alan.

[BlueTR3A-5EKT]

In the old days....

C&B used to stock Plastigage for the use in the workshop and sale to Rover V8 customers. Very useful for setting/checking pump gear end float. (Think PI fuel pump)

Moss mention it in the MG'T' series instruction https://www.moss-europe.co.uk/media/pdf/435-088_instructions.pdf

 

Peter W

[oldtuckunder]

Its quite reassuring when you get a crank back, and with plastigauge every journal comes out at what you asked for. Once when in a hurry (never a good thing) I even took the block,rods and shells to the machine shop and as the crank was finished plastigauge'd it on the spot, the result was that three journals went back on for a final polish.

 

Alan

[Waldi]

Thanks Alan,

I felt like a little boy with a present. Theory is one thing, but if it matches in practice, thats very rewarding to me.

Cheers,

Waldi

[alaric]

Plastigage is a modern development of the traditional technique of 'Taking Leads', to measure clearance while scraping in a white metal bearing.

Plastic replacing soft lead wire.

As plastic is now seen as a major environmental hazard, should we revert to lead?

Jason Wright demonstrated the 'new stuff' at the excellent Engine Seminar at TRGB last weekend as previously mentioned.

Edited April 1, 2018 by alaric

[RogerH]

....and you are saying lead isn't dangerous .........

 

Roger

[john.r.davies]

Depends on how fast its going.

[RogerH]

[Waldi]

Yes:

Slow lead kills you slowly.

Fast lead very quick.