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Author Topic: Pistons - Thrust Side?  (Read 259 times)
Rod
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« on: October 02, 2019, 10:46:14 AM »
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Hi All,

I am in the process of cleaning up a grey (Replacement motor - Vic Police Engine Number). I put this low milage motor together about 30 years ago and have done very little milage since. I have documented a knock in another thread. In cleaning up I have paid particular notice of the pistons on each rod. I have read the manual and indicated that when installing pistons back on the rods that the thrust side of each piston needs to point towards the oil bleed holes ie: camshaft side.

While I am a while away from doing the rebuild I felt that I had to clarify the notion of thrust side of the piston. I searched the internet and this forum but are struggling for an answer. Each piston is stock (3 1/16) with no markings evident on the crown indicating the forward direction of the piston. When I looked at the markings on the inside of each skirt there is the following - "4" and "Holden". What has led me to this query is that when I looked at the position of each pistion in relation to the bleed holes, three pistons had "4" and three pistons had "Holden" facing toward the bleed holes. This to me would indicate that 3 of the pistons are facing the wrong way, but which 3? I suspect when I had new gudgeon installed 30 years ago, the machine shop didn't pay particularly attention to the postion of the pistons as there was no markings on the crown indicating forward positioning.

I understand there is an offset in the piston pin to accomidate the thrust but I am struggling with this. Are there any grey guru's who may be able to indicate which side of these pistons would be the thrust side.

Thanks in anticipation.

Rod

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ardiesse
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« Reply #1 on: October 02, 2019, 12:53:11 PM »
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Rod,

Red motor pistons have offset pin bores, but grey motor pistons don't.  The very earliest grey motor pistons have T-slots in one skirt, and need to be installed the right way around.  Your pistons, being 3-1/16", won't have the T-slot.  My impression is that later grey motor pistons may be installed either way around.

Once the motor's been in service, I'd suggest you leave things as they are.  The thrust side of the piston will develop a larger wear mark on the skirt than the non-thrust side.  You can use the size of the wear marks as a guide to determine if the previous builder put some of the pistons back on the conrods the "wrong" way around.

Rob
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Rod
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« Reply #2 on: October 03, 2019, 09:44:35 PM »
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Thank you Rob for your feedback. With your exceptional knowledge and experience of greys, it was no suprise to me that you responded. I have learnt so much following your grey motor threads.

I was hoping to hear what you had to say in that it doesn't matter which way the pistons face. What me led me to the question was the instructions in the manual and having fitted NOS Pistons to another grey (stock bore) which had a dot on the crown to indicate to be placed in a forward position. I was also have a set of 40thou oversized pistons (holden made) which also have a dot on the crown. All of this made it a little confusing for me.

Over at this ongoing thread (Re: Engine Noise) of the motor I am currently looking at I think I had finally found the cause of an ongoing knock (reversed clutch plate). What has confused me in all of this though is that when I disconnected No.6 spark lead to isolate the cylinder, the knock disappeared. I was wondering if it was initially "piston slap" which could be caused by having the piston 180 degrees out.

Anyway I now feel at ease in putting the pistons back in the postion they came out of after your feedback.Once again many thanks.

Cheers

Rod
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ardiesse
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« Reply #3 on: October 04, 2019, 09:51:11 AM »
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Rod,

I'm doing all this from memory, without having consulted the shop manual.  So it's wise to double-check.
Standard size 3-1/16" Holden pistons have a small counterbore in the centre of the piston crown, I remember, but I'm not certain about oversize pistons, or whether all GMH pistons carry the same markings on the crown.

Rob
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my8thholden
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« Reply #4 on: October 05, 2019, 06:28:04 AM »
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Rod ..Something to note when looking at used parts to access their condition or re servicibility or why it has worn in a certain way ,sometimes unexplained ,eg why some cam followers wear differently than their mates in the same engine ,and also parts that are 60 plus years old may have been mixed up in every conceiavable way.With regards to pistons ..Most if not all alloy pistons are circular around the ring lands and oval around the skirt ,so you will notice wear marks on the skirt at 90 deg to the pin ,and usually a little more evident on the thrust side .Some times used pistons can be in good condition ,but when you put them in the bore down to the pin they rock sideways against the thrust side of the skirt ,excessive " slap "..There is a tool called a piston expander  ..some engine shops may have one of these ,it hammers the skirt back out from the inside ,also Holden pistons made by ACL had different color coding on each set of 6 pistons ,eg RED , BLUE , YELLOW ,this was the bore honing clearance for that particular set ,it was tiny ,and would not be evident on old pistons ..one thing you could do is weigh your final set " must be a set " of 6 pistons ,and remove a little excess metal internally ,till all same weight as the lightest ..also do the con rods ,your piston pin should slide firmly but not tight or jerky into the piston with thumb pressure and a little oil .do your weights ,you may be surprised ,post them here ...Vern 
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my8thholden
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« Reply #5 on: October 08, 2019, 09:18:54 AM »
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Rod ..The pistons in my engine are Repco pistons 3 1/16" plus .030 thou ..The part number stamped on the crown does not appear in old Repco catalogue ,and has the old Repco symbol on them ,so bit of mystery ,I checked under Chev listings ,not there ,I don't have a old numerical listing ,they don't appear special in any way ,standard rings and pin..but because there is no indication of " front " on them ,and no other markings or design features ,eg " T " slot in the skirt ,I put them in the engine with the part number and Repco symbol to the front ,as it is stamped in the same position on each piston ,ill post an image if you wish to see that ..Some English pistons for English engines had a instruction to fit pistons with stamped numbers to front ..it would be handy if all pistons had a dot or arrow indicating front ,you can always put your own on for future reference when you pull the engine down after 200,000 miles.cheers Vern
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