4.8L / 96MM Piston problems

4.8L / 96MM Piston problems

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Discussion

debaron

Original Poster:

866 posts

196 months

Thursday 27th April 2017
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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

neal1980

2,574 posts

238 months

Friday 28th April 2017
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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 smile


Jhonno

5,762 posts

140 months

Friday 28th April 2017
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All you need to do is get the bores machined properly/to spec..

RobXjcoupe

3,151 posts

90 months

Friday 28th April 2017
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You bore the liners to suit your new pistons, short skirt isn't an issue it's the dimension and tolerance the liners are bored to.
What size are you boring the liners to?

debaron

Original Poster:

866 posts

196 months

Friday 28th April 2017
quotequote all

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??

debaron

Original Poster:

866 posts

196 months

Friday 28th April 2017
quotequote all
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 smile

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

RobXjcoupe

3,151 posts

90 months

Friday 28th April 2017
quotequote all
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 piston

LongBaz

2,087 posts

216 months

Friday 28th April 2017
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Speak to Roland at ACR he builds lot of 4.8 motors, I have one in mine.

ClassicChimaera

12,424 posts

148 months

Friday 28th April 2017
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RobXjcoupe said:
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 piston
If I knew that I'd be rich biglaugh

phazed

21,844 posts

203 months

Saturday 29th April 2017
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Most engine builders keep, "the knowledge "to themselves.

Quite often it is difficult to get the correct information from these specialists. They keep it to themselves for obvious reasons.

I would've thought the manufacturer of the piston would give you the information you require .

RobXjcoupe

3,151 posts

90 months

Saturday 29th April 2017
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Reading through again to make sure. We are talking about the bore size tolerance to enable the piston rings to work correctly?

RobXjcoupe

3,151 posts

90 months

Saturday 29th April 2017
quotequote all
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?

macdeb

8,492 posts

254 months

Saturday 29th April 2017
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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

12,424 posts

148 months

Saturday 29th April 2017
quotequote all
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.

macdeb

8,492 posts

254 months

Saturday 29th April 2017
quotequote all
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.

RobXjcoupe

3,151 posts

90 months

Saturday 29th April 2017
quotequote all
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. smile
In .000mm (microns) 1 thou is roughly .025mm

Edited by RobXjcoupe on Saturday 29th April 19:42

debaron

Original Poster:

866 posts

196 months

Saturday 29th April 2017
quotequote all
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. smile
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

debaron

Original Poster:

866 posts

196 months

Saturday 29th April 2017
quotequote all
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.

RobXjcoupe

3,151 posts

90 months

Saturday 29th April 2017
quotequote all
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 smile


Edited by RobXjcoupe on Saturday 29th April 19:51

Boosted LS1

21,165 posts

259 months

Saturday 29th April 2017
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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.