Rangie46 Posted October 15, 2015 Share Posted October 15, 2015 Picked up another rv8 off eBay the other day a 4.6 bottom end for 100 quid. ( think I have an addiction) Started to tear it down today and it looks like it's had a rebuild resently, as you can see from the pics below the conrods have been punched to mark their location and the bearings look brand new hence the question, from the pic below I think the main bearing maybe +10 oversize can any one confirm? Still need to pressure test the block but even if it's porus I can still put the crank and rods into the 4.0, I also have a spare set of 4.0 Pistons should I use these on the 4.6 rebuild or stick with the 4.6 ones? Quote Link to comment Share on other sites More sharing options...
need4speed Posted October 15, 2015 Share Posted October 15, 2015 Great find! Use the 4.0 Pistons and gain some compression.. Quote Link to comment Share on other sites More sharing options...
garrycol Posted October 17, 2015 Share Posted October 17, 2015 It depends - if you have 4.6 Hi Comp pistons then that should give the highest comp ratio but 4.0 low comp pistons will give about 9.2:1 in a 4.6 engine where 4.6 low comp pistions will still only give 8.3:1 Garry Quote Link to comment Share on other sites More sharing options...
need4speed Posted October 17, 2015 Share Posted October 17, 2015 No I don't think that's correct. I was told that the 4.0 pistons and the 4.6 hi-comp pistons gave 9.75:1 compression. You also have to take into consideration whether tin or composite gaskets are used. Also, 4.6 hi-comp pistons are a little more hard to come by here in uk so I still think the 4.0 Pistons is a more realistic proposition Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted October 17, 2015 Share Posted October 17, 2015 Agreed, n4s, that's my understanding too. I have a set of low mileage 4.0l low comp pistons for sale, in case anyone interested.... Quote Link to comment Share on other sites More sharing options...
garrycol Posted October 17, 2015 Share Posted October 17, 2015 I can only relate my own experience (not hearsay) where I looked at this in rebuilding my 4.0 as a 4.6. In doing the calcs with my engine and fitting 4.0 low comp pistions into a 4.6. I believe a standard HC 4.6 is either 9.5:1 or 9.35:1 (different advice in different sources) - LC engines are 8.13:1. In working out what pistions to use in my rebuild (i had 4.6 and 4.0 LC pistons) the engine builder calculated theoretical compression ratio of the 4.0 LC pistions in the 4.6 as 9.2:1 - real numbers not hearsy. Netspeed if the 4.0 pistons and the 4.6 hi-comp pistons gave 9.75:1 compression why would LR use different pistons - easier and simpler to just use the same piston for both applications which they dont. Garry Quote Link to comment Share on other sites More sharing options...
elbekko Posted October 18, 2015 Share Posted October 18, 2015 From the RAVE: 4.0: 4.6: Quote Link to comment Share on other sites More sharing options...
need4speed Posted October 18, 2015 Share Posted October 18, 2015 Seems to be lots of conflicting advice here. Garry, if you call what I was told by V8Developments heresay then I guess it was heresay.... With that being said, it appears as though RAVE is stating something different from what I was told as well. It may be then that my engine will not have the compression that I had hoped for I had hoped that I would have slightly higher than 9.75 compression working from the premise that I was starting from 9.75 and then using tin gaskets rather than composites. Quote Link to comment Share on other sites More sharing options...
garrycol Posted October 18, 2015 Share Posted October 18, 2015 Irrespective of all that - for the OP if 4.6 Hi Comp pistons are not readily available (nor 4.0 Hi comp) I think we all agree to go 4.0 low comp pistons over 4.6 low comp pistons in a 4.6. Quote Link to comment Share on other sites More sharing options...
Rangie46 Posted October 18, 2015 Author Share Posted October 18, 2015 Cheers guys it seams that who ever did the rebuild on this 4.6 used the 4.0 pistons aswell, as I cannot see any difference between the spare 4.0 pistons I have and the ones that are in the block, and I remember seeing a pic somewhere showing the difference in the dish of the 4.0 and 4.6 pistons which is easy to see, so looks like I have had right bargain, hopefully the block will pass the preasure test as I can't see anyone putting money and time into a rebuild of a knackerd block, will update later next week once I test the block. Quote Link to comment Share on other sites More sharing options...
garrycol Posted October 18, 2015 Share Posted October 18, 2015 Just remembered - there is one difference - the gudgeons on the 4.6 pistons are thicker than the 4.0 pistons - that is, the outside diameter is the same but the 4.6 gudgeons are made of thicker walled tube. Not sure whether it is an issue but when I was rebuilding my engine I took the 4.6 piston gudgeons out and put them in the 4.0 pistons. Quote Link to comment Share on other sites More sharing options...
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