Do I need new pistons?? - Alfa Romeo Forum
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(Post Link) post #1 of 13 Old 21-06-09 Thread Starter
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Do I need new pistons??

My pistons on my GTV Twinnie took a right hammering from the valves when the cam belt slipped.



The question is, will this matter, do I need new ones or not? Any help or advice gratefully recieved
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I'd say not if its just the top surface after all they are sold pre groved for valve clearance anyway, but pistons have got to be easy to come by for peanuts.
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The ones on my 155 look worse than that thanks to the previous owner. They still work fine although I think it does effect the performance a bit.
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I would say that they will be ok, however I have heard of the valve-piston impact closing the ring gaps slightly on occasion, so it might be worth measuring them.
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It is good to grind a bit that marks to get more even surface. I suspect that you will skim the head anyway so the CR will be more or less the same.
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Yes, remove any rough edges on the "burred" areas and they'll be alright. What you don't want is anything that can attract heat (raised areas/"peaks" normally) or they may cause pinking/hot-spots (ultimately in extremis piston melting).. but yours look re-usable.


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(Post Link) post #7 of 13 Old 24-06-09 Thread Starter
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Thanks for the replys guys, I am going to clean them up and reuse them!
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personally i wouldnt,, the problem lies not in the piston but its balance, the damage throws off the balance of the piston,, also the collision can have weakened the small end bearing ,possibly the big end bearing but unlikely., so to be safe id say replace the piston con rod and bearings. or id say in up to 3000 miles it may go bang.
im sure there are many owners who have had a belt snap and done the repair,, only to have an engine blow.. normally no engine oil warning light will come on, indicationg that the tapping / knocking was from the small end bearing on the conrod.

hope this helps
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big end bearings are fubarred in 80% of the cases with broken cambelts, small ends are uncommon to go...
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(Post Link) post #10 of 13 Old 16-08-09 Thread Starter
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i have reused the old pistons. I fitted new rings and new big end shells but i couldn't find one person who thought i should change the pistons (until now!) head is being skimmed at the moment so hopefully we are not too long off knowing if it was a good decision!
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sorry, just my experience as an engineer and ive had 50 plus cars and bulit many engines. big end failure on the car would give you an indication through loss of oil pressure, small end bearing,,will give no such warning other than the tap tap bang,,, just thought it was best to give my thoughts
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(Post Link) post #12 of 13 Old 16-08-09 Thread Starter
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surely big end failure can be caused by low oil pressure, not cause low oil pressure? After taking loads of advice from people with experiance of this engine, the concensus was to change the big ends, not one person mentioned the small ends and tbh i have never heard of this on any marque after a cam belt failure. Running low oil perhaps but not cam belt. Anyway, its always good to hear a different opinion!
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causes low oil pressure as there is no resistance for the oil in the bearing shell,, once this goes oval or hammered the oil just passes through with no resistance to the sump,, hence pressure drop
hopefully though u dont have any probs
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