Piston crown numbers.Does ID20 mean anything?

nickm · Post by **nickm** » Wed Jul 20, 2011 12:31 pm

I have been given a 3.9 engine out of a Range Rover which I want to rebuild.It was apparantly using loads of oil so I am wanting to rebore it.However the piston crowns have ID20 stamped on and I am worried that means it has already been overbored 20 thou.Could that be the case?if so can you get 40 thou oversize pistons for the 3.9?Thanks,Nick

DaveEFI · Post by **DaveEFI** » Wed Jul 20, 2011 2:35 pm

Is the bore wear actually bad enough to require a re-bore? These engines can do an enormous mileage before it's needed.

kiwicar · Post by **kiwicar** » Wed Jul 20, 2011 2:39 pm

Hi
Yes it sounds like they are 20 over. Your choices are 30 over pistons if you can find a set and 10 thou rebore will clean them up enough, 40 thou Rover pistons, but those are uncommon to say the least. Or chevy 305 pistons (+36 thou overbore on standard rover), to use these you either have to ream out the rover little end or use chevy rods and skim 20 thou off each rod (it has been written up on here a few times what to do to use chevy rods you would also need to chose the right deck height for the pistons. Chevy pistons are cheep for what you get, especially if you want forged pistons, likewise chevy rods are much cheeper than uprated rover rods but is it is just a "cooking" rebuild I would just look for a standard bore block and rebore that.
Best regards
Mike

nickm · Post by **nickm** » Wed Jul 20, 2011 3:08 pm

Thanks for the replies.Chevy pistons interesting idea.The bores do not have a lip at the top of the bore so wonder if they are not worn but the honing looks peculiar ie circumferential marks rather than 45degree so I wonder if a rehone with new pistons might suffice?I suppose I need to get it all measured up at an engine shop.Nick

kiwicar · Post by **kiwicar** » Wed Jul 20, 2011 3:23 pm

Hi
Measure them up is a good plan. As a general rule Rover bores last a long time, you may find that it just needs a re-ring and hone. This is especially likley if there is no apreciable lip top and bottom, you would be looking at areas of the bores that were very "pollished" and they ware pretty unevenly when they are worn out. I would measure first then decide whay to do.
Best regards
Mike

minorv8 · Post by **minorv8** » Thu Jul 21, 2011 5:03 am

I have come across the IDxx markings on Hepolite pistons. They are NOT necessarily the size of the piston. Usually oversizes are marked +020 or +0,50 if they are e.g. from Mahle.

Measure the bore to be sure.

nickm · Post by **nickm** » Thu Jul 21, 2011 8:56 am

Have measured the bores now and they seem to be all pretty much spot on 94mm.Will it be ok to have them rehoned properly or would it be best to rebore to 20 thou?

kiwicar · Post by **kiwicar** » Thu Jul 21, 2011 9:21 am

Hi
You need to measure for how ovel the bores are, and how muuch taper there is, basically 6 measurments, 10% 50% and 90% down the swept area of the bores, in line with the block and at 90 degrees and check it against rover specs. If you can do a re-ring and hone it is cheeper than new pistons and much simpler as you don't have to remove the pistons off the rods.
Forgot, off my list of piston options are ford Mod engine ones (5.7 litre engine I think), 3.7" bore, flat top, forged, light and cheep (Ian Stewart found them and they are nice

) but only really suitable for a high compression build, or if you use buick 300 heads.
Best regards
Mike

DaveEFI · Post by **DaveEFI** » Thu Jul 21, 2011 10:07 am

nickm wrote:Have measured the bores now and they seem to be all pretty much spot on 94mm.Will it be ok to have them rehoned properly or would it be best to rebore to 20 thou?

The most usual indication of bore wear is a lip at the top - where the rings don't bear on. If that is within tolerance, a hone and new rings will be very much cheaper than a re-bore and new pistons - and just as good.