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3 Rings Vs 4 Ring Pistons

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Best Answer Spider , 31 March 2019 - 07:22 AM

3 vs 4 Ring Designs - Generally, 3 Ring types have less drag / friction and so are generally more efficient.

 

The other thing to look for in regards to Rings are a 3 or 4 piece Oil Control Ring, they control the Oil on the Bores WAY better than any one (or two) piece Oil Control Ring.

 

While I haven't looked at the late UK Press Fit Rods in detail, we had them in Aust in the Mid 60's to 1970 in our locally made 998 Engines. These were the same Rod as the floating Pin type, only the left the Bush out and the Pin (which was bigger than the floating type) then pressed in to the Rod. It might be worth looking at these late UK Types to see if in fact they did the same thing. If so, then all you need do is fit the Bushes to the Little Ends, have them Honed to a fined size and your in business.

 

It appears there's very very limited choice in the Press Fit type Pistons for small bore engine.

 

<Edit: - I've just looked up the Pin Sizes between the Floating Pin and Press fit types - they both appear to use the same size pin. Can someone confirm if in fact they are ? >

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#1 BaronVonchesto

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Posted 31 March 2019 - 05:15 AM

So I've made several posts about pistons of late, this will hopefully be the last one!

 

Anyway in trying to decide what pistons to buy, there seems to be a fair bit of variance in terms of cost and availability by supplier. Many suppliers have pistons that use 4 rings (looks like 3 compression rings and an oil ring). The original Mini pistons I have use 3 rings instead.

 

Any concern with buying pistons that have 4 rings instead? Would they somehow be longer or something, or would they not fit to the standard conrods?

 

Also my pistons are the press fit type but most of whats available seem to be the circlip type. Does that mean that my options are much more limited or can I actually use the circlip type pistons on my press fit conrods?



#2 Spider

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Posted 31 March 2019 - 07:22 AM   Best Answer

3 vs 4 Ring Designs - Generally, 3 Ring types have less drag / friction and so are generally more efficient.

 

The other thing to look for in regards to Rings are a 3 or 4 piece Oil Control Ring, they control the Oil on the Bores WAY better than any one (or two) piece Oil Control Ring.

 

While I haven't looked at the late UK Press Fit Rods in detail, we had them in Aust in the Mid 60's to 1970 in our locally made 998 Engines. These were the same Rod as the floating Pin type, only the left the Bush out and the Pin (which was bigger than the floating type) then pressed in to the Rod. It might be worth looking at these late UK Types to see if in fact they did the same thing. If so, then all you need do is fit the Bushes to the Little Ends, have them Honed to a fined size and your in business.

 

It appears there's very very limited choice in the Press Fit type Pistons for small bore engine.

 

<Edit: - I've just looked up the Pin Sizes between the Floating Pin and Press fit types - they both appear to use the same size pin. Can someone confirm if in fact they are ? >


Edited by Moke Spider, 31 March 2019 - 08:48 AM.


#3 DeadSquare

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Posted 31 March 2019 - 10:33 AM

The pin size is constant, it is the little end bore/bush that varies.



#4 unburntfuelinthemorning

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Posted 31 March 2019 - 10:50 AM

According to Des Hammil's book for tuning the 998 the gudgeon pin sizes are different and not interchangeable.

 

At the beginning of the chapter on con rods he quotes the interference fit pin as being 0.6121" nominal diameter and the floating pin at 0.6247". 

 

At the end of the chapter he provides this information:

 

Piston pin/gudgeon pin/little end bore diameter: 0.625"  (early A+)

                                                                             0.6134" (later A+)

 

Average piston pin diameter: 0.6244" (early A+)

                                               0.6124" (later A+)

 

Interference fit of connecting rod little end: 0.0008" to 0.0013" (late A+ rods only)

 

So is this correct?



#5 DeadSquare

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Posted 01 April 2019 - 08:03 AM

Are you sure that those figures are the right way round ?.  Surely a floating pin would be smaller than an interference fit ?.

 

I wouldn't like to be the man told to push a 12.5 thou oversize pin through a little end.

 

1.3 to.8 thou is more like it.

 

As far as I know, all the production small bore engines, ie: 948 / 848 / 997 / 998 / 1098 have a .625" pin.



#6 unburntfuelinthemorning

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Posted 01 April 2019 - 09:24 AM

Well of course you wouldn't but he's saying the earlier and later rods/pins are not interchangeable because of the different pin sizes.

 

The figures are correctly quoted from the book.  I have no idea if the figures published in the book are correct or not. 

 

There are some discrepancies for instance on the later A+ figures the pin diameter is quoted as being 0.6214" and the little end bore diameter as 0.6224 so the pin is 0.001" smaller than the little end.  Not much of an interference fit there is there, surely the pin should be 0.001" larger than the little end bore?!

 

Then he quotes different sizes for the pins at each end of the same chapter.

 

I like Hammil's books but there does seem to be some stupid mistakes in them.

 

Anyway Baron, get bushes fitted to the rods as Moke Spider says or get hold of some earlier rods with the bushes already fitted or get the later pistons.



#7 BaronVonchesto

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Posted 01 April 2019 - 09:55 AM

thanks for the additional replies. lets take the discussion over to the other post i made specific to press fit vs circlip conrods:

http://www.theminifo...ss-fit-conrods/

 

i think it's fair to say that 3 ring pistons are preferable to 4 ring ones, so i think we can close this topic.







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