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Piston Dilemma


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#1 skoughi

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Posted 05 September 2015 - 04:49 PM

I'm checking things over regarding placing an order for rebuilding my 1330 engine and I'm in a quandary regarding pistons and rings. I have four ring pistons just now that have as far as a quick check goes has an 11.5 dish. I've measured the ring land gaps and they're coming up as 0.071" 0.067" 0.067" 0.158", so I'd probably need to call minispares to see if these gap sizes are still ok. If they're past their best then I'll need to look for new pistons but I'm unsure what to go for. This is a budget build (this seems to be quickly going out the window!) so I'm wondering what piston to maybe go for compression wise, either the P21253-60 9.75cr or should I go with the lower P21251 8.8cr? 



#2 ACDodd

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Posted 05 September 2015 - 05:48 PM

Put old ring in groove, if you can get a 0.004" feeler blade between the ring and groove, you need new Pistons. It's the inspection that sets the budget not the money you actually have available. I true budget build actually means that the engines does not require many parts or machining. In most cases with mini engines, owners leave the engines so long before overhaul work that most items need replacing or machining.

Ac

Edited by ACDodd, 05 September 2015 - 05:51 PM.


#3 skoughi

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Posted 05 September 2015 - 05:54 PM

Thanks for the info regarding checking the ring wear, I take it that I can measure this with an old ring? I like what you said about the budget!



#4 skoughi

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Posted 05 September 2015 - 06:03 PM

Okaaay, I nipped out and checked a couple of pistons and found I could get .020 - .021 between old rings and pistons! I guess that means I can add new pistons to the list whenever I like!!



#5 skoughi

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Posted 05 September 2015 - 06:16 PM

I guess what I mean by budget is not trying to get away without replacing anything but replace and repair what's required but not buying top end parts or modifying the engine too much, if you know what I mean!. I think if I had the choice then I should probably be better off sourcing a good short motor, but then if I can get this engine back together and burning less oil than petrol then I'll feel a whole lot more smug!!



#6 skoughi

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Posted 05 September 2015 - 07:26 PM

Any views on what piston to maybe go for compression wise, either the P21253-60 9.75cr or should I go with the lower P21251 8.8cr? 



#7 Spider

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Posted 05 September 2015 - 08:07 PM

If the head and block haven't been skimmed, then I'd suggest going for the 9.75's

 

 they're coming up as 0.071" 0.067" 0.067" 0.158",

 

The top groove is worn, bin them.



#8 skoughi

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Posted 05 September 2015 - 08:43 PM

Thanks moke, after following AC's advice it can clearly be seen that they've had their time. My worry regarding C.P. is with standard octane unleaded (to be honest I'm not sure what octane our local pumps sell) and any problems this can have with the A series. I'm thinking I won't be wanting a C.P. any higher than 9.75 will I?



#9 Spider

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Posted 05 September 2015 - 09:46 PM

Regarding the CR, what Cam will you be running? 



#10 skoughi

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Posted 06 September 2015 - 09:21 AM

Kent 266



#11 Spider

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Posted 06 September 2015 - 10:27 AM

OK, based on Kent's published Inlet Closing Angle of 560 and allowing for 20 initial advance (to allow for chain stretch), a static CR of 9.75:1 is very safe, even running on 91 RON fuels, in fact you can very comfortable up to 9.95:1.

 

If you wanted to use 97 RON, the CR can safely go to 10.31:1 static CR

 

If you wanted to use 98 RON (is this available in the UK?), you can safely go to 10.42:1 static CR

 

No doubt AC will be along to suggest something else!


Edited by Moke Spider, 06 September 2015 - 10:30 AM.


#12 skoughi

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Posted 06 September 2015 - 11:53 AM

Guys like yourself are invaluable for numpties like me!! Thanks very much for the info, I'll need to ask at my local pumps what they actually sell. I'll go for the higher comp pistons then, cheers!



#13 carbon

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Posted 06 September 2015 - 07:04 PM

I think you may struggle to find anything better than 95 octane at the pumps in Shetland, or anywhere north of Inverness for that matter.



#14 skoughi

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Posted 06 September 2015 - 07:24 PM

You could well have a valid point there Carbon! I'm guessing there's a lot of fuel octane booster that gets sold up here!



#15 ACDodd

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Posted 07 September 2015 - 12:22 AM

I like 9.75:1 with the 266 cam, I have my best results at this static CR setting, dial the cam it at 104.

 

AC






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