4.8L / 96MM Piston problems
Discussion
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
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??
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 bothYou 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
debaron said:
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??
What clearance is given for a standard 4.6 engine and a tvr spec 5.0? I'm sure the 5.0 is a short skirt pistonhttps://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??
Having a read online, the bore tolerance is different on the type of power out you want to achieve and the type of piston you are using due to its thermal expansion properties. 35thou seems to be a very standard engine using bog standard pistons. 15thou clearance is used on special type material pistons as less movement due to heat but a higher clearance is needed if the engine is say forced induction. You also have a bore tolerance top to bottom as well as bore to piston and the bore to piston tolerance increases the bigger the diameter piston is.
Personally I would stick to a tuned normally aspirated engine as tolerances are readily available for the amount of heated produced within the bore.
If say you are going forced induction all I can suggest is find the specs of a similar output factory manufactured engine and see if the materials used match yours and go from there.
I can ask a couple of engineers here and possibly find out the bore to piston tolerance on the jlr 5.0 engines we make?
Personally I would stick to a tuned normally aspirated engine as tolerances are readily available for the amount of heated produced within the bore.
If say you are going forced induction all I can suggest is find the specs of a similar output factory manufactured engine and see if the materials used match yours and go from there.
I can ask a couple of engineers here and possibly find out the bore to piston tolerance on the jlr 5.0 engines we make?
Manufacturers have to safeguard themselves and so give a generous clearance recommendation. That spec of piston is very strong (I have some) but have different expansion rate 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.
Edited by macdeb on Sunday 30th April 12:12
ClassicChimaera said:
Does ring (gapping ) also come in to all of this when creating a good seal.
I'm sure the engine builders will offer guidance on your specs but not necessarily online.
Yes. Also, check out 'puma racing' threads on the subject as he is one guy that used to be on here that I respect the opinion of. Another important thread to look at is his method of running in an engine as so many get this so wrong or even forget about.I'm sure the engine builders will offer guidance on your specs but not necessarily online.
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 NAIn .000mm (microns) 2 thou is roughly .025mm (25microns)
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.H7 is the bore, h6 is the piston.
You need to measure the piston for its exact size. Then the H7 gives the tolerance of +0.035mm -0.000mm of the exact size of your piston. Each piston will be machined with a tolerance so best to mark each piston so you know exactly what size to apply the tolerance to and obtain a machining finish size per bore and piston.
35-45 thou is a monstrous size in toolmaking. My tolerance chart is in microns of a mm so don't confuse the two.
Also check if each piston has the gudgeon pin in the centre or not as when it comes to building they need fitting the right way round if off centre.
Monday I will have a chat to confirm the tolerance we use here and see how that compares to your manufacturer quoted figure
You need to measure the piston for its exact size. Then the H7 gives the tolerance of +0.035mm -0.000mm of the exact size of your piston. Each piston will be machined with a tolerance so best to mark each piston so you know exactly what size to apply the tolerance to and obtain a machining finish size per bore and piston.
35-45 thou is a monstrous size in toolmaking. My tolerance chart is in microns of a mm so don't confuse the two.
Also check if each piston has the gudgeon pin in the centre or not as when it comes to building they need fitting the right way round if off centre.
Monday I will have a chat to confirm the tolerance we use here and see how that compares to your manufacturer quoted figure
Edited by RobXjcoupe on Saturday 29th April 19:51
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.
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