4.8L / 96MM Piston problems

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About to start an engine build for the Chim using 2618 alloy 4.8L forged pistons. Been looking around and there are two threads about 4.8L builds that have both gone bad using the same pistons I have. Seems it might be down to the bore clearance being too large coupled with short side skirts.

Any advice on what way best to go? Engine not linered yet so not too late to go back to 94mm cast Rover ones but means I'll prob not get to put the supercharger on....

anyone running a 4.8L that hasn't had problems??


Ta

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The manufacturer calls for 0.0035 clearance - same as the fella here who had his build go bad...


https://www.pistonheads.com/gassing/topic.asp?h=0&...


Note PumaRacing's comments about that clearance causing piston slap - so this is exactly what I want to avoid....

But not sure who is going to advise on a better clearance??

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neal1980 said:

Are you sure there the same brand ??

You will have nothing to worry about if you get the tolerances correct. I certainly wouldn't give up having forged pistons over cast. Build it yourself correctly then you know its done right

Hi Neal - yep double checked model numbers which can be seen on both

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RobXjcoupe said:

Picture below shows standard engineering bore and shaft clearance fits. If you are quoted a H7 h6 tolerance for example the chart will tell you what finished dimension is needed.
In .000mm (microns) 2 thou is roughly .025mm (25microns)

Excellent thanks Rob - Dom at Power is doing the machining so will chat to him too and get his advice. Not sure how the H tolerance applies to mine.. Manufacturer says 0.0035 for boosted, 0.0045 for NA

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macdeb said:

Manufacturers have to safeguard themselves and so give a generous clearance recommendation. That spec of piston is very strong (I have some) but expand less with heat and are probably overkill for the road TBH. Close the clearances down a touch and have bores finish honed to that. I'd also use traditional rings and not total seal.

Thanks Mac - May I ask what clearance you went with? I know they are a bit full on (and pricey too) but they seem to be the only forged piston that works easily with the standard 4.6 setup - so if you're going big boost it's them or melted cast ones!! I have the Hastings standard rings too.

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Boosted LS1 said:

This seems to be confusing. 35 thou is massive for piston to bore clearance, assuming you mean .035". Some pistons can run as tight as .0005" whilst forgings may be happier at .005". I also think that once the piston's expanded, that's it. It won't keep on expanding as the skirts are shedding heat.

Yep - it's 3.5 thou = 0.0035 clearance as specified by manufacturer

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Boosted LS1 said:

Does the manufacturer say where to measure the piston?

Yep - at the dial point (.500” from bottom of skirt up and at 70F) piston should measure 3.7775” -3.7780”.
Recommended clearance for this bore and boosted - .0035”
Final bore size: 3.7775” + .0035” = 3.7810”