can 4.6 block be rebored?

conrod · Post by **conrod** » Mon Apr 29, 2013 10:24 am

I have a 4.6 engine that I am rebuilding to put in my race car tow vehicle (LDV van). We have measured the block, and found it has too much wear to hone and re- ring. This means either a rebore to first oversize, or a new set of liners and back to std size. As it is on the std bore this would mean finding a set of +.020" pistons and rings (these seem rather scarce?) and boring to this oversize. There was no evidence of a cracked block before. Won't be looking for big hp, (250hp max, but with good torque and I will be running it much cooler than in the RR (around 80°C) so that might help things too. I have been told overboring might weaken the liner, as it is already very thin.

I really don't want to spend a lot of money putting top hat liners in if I can avoid it. So my question is, would going +0.020" be too risky in one of these engines, or am I safe?

Conrad

kiwicar · Post by **kiwicar** » Mon Apr 29, 2013 10:34 am

HI
+20 is fine, +36 is also a common option (chevy 305 pistons) and therefor +40 is also viable wthout new liners.
Best regards
Mike

conrod · Post by **conrod** » Mon Apr 29, 2013 4:15 pm

thanks Mike, good to know:) I will try to find some oversize pistons.

cheers Conrad

ChrisJC · Post by **ChrisJC** » Mon Apr 29, 2013 8:30 pm

Land Rover do not recommend reboring the block. This is why there are no genuine parts oversize pistons.

Of course you might be able to do something as Mike says.

Chris.

DEVONMAN · Post by **DEVONMAN** » Mon Apr 29, 2013 10:03 pm

Real Steel do +20 4.6 pistons

kiwicar · Post by **kiwicar** » Tue Apr 30, 2013 8:25 am

"Land Rover do not recommend reboring the block. "
Hi Chris
don't you think that might be a way of selling more blocks? call me an untrusting sole!
Best regards
Mike

conrod · Post by **conrod** » Tue Apr 30, 2013 8:36 am

thanks for the replies guys, I will probably go with Real Steel, and get one of their camshaft kits while I am at it.

I do understand why LR do not recommend reboring, and know it is a risk, but am prepared to give it a crack (bad choice of words I suppose!)

Does anyone know the volume of the dish in the 4.0 piston, and the volume of the dish in the 4.6 piston? I know the 4.0 is less and will bump the CR up, but are we talking 10:1, 11:1 or what? Initially I will run on petrol (96 octane) but may possibly fit LPG in the future, so a higher CR will certainly help there. But I don't want to go too high and make it unusable on pump gas.

cheers Conrad

minorv8 · Post by **minorv8** » Tue Apr 30, 2013 9:19 am

4 litre piston dish is 13,2 cc and 4,6 litre piston is 22,3 cc.

Piston swap would bring the CR up about one whole point i.e. from 9,35 to 10,3. Most likely a bit less but can´t remember exactly.

minorv8 · Post by **minorv8** » Tue Apr 30, 2013 9:24 am

Land Rover do not recommend reboring the block. This is why there are no genuine parts oversize pistons.

I remember seeing a Volkswagen VR6 engine in bits and pieces at local engine shop a few years back. It was worn and needed a rebore but there were no OEM or aftermarket oversize pistons available. Maybe factories calculate that people buy new cars after a few years so there is simply no need for engine rebuild especially if the body itself slowly corrodes away. Basically hte engine lasts the lifetime of the car itself.

conrod · Post by **conrod** » Tue Apr 30, 2013 9:52 am

minorv8 wrote:4 litre piston dish is 13,2 cc and 4,6 litre piston is 22,3 cc.

Piston swap would bring the CR up about one whole point i.e. from 9,35 to 10,3. Most likely a bit less but can´t remember exactly.

awesome, just what I was looking for!

by my calcs. that will give a CR of 10.66:1 when 4.0 piston is fitted to 4.6 engine. Might be a touch high for my requirements?

minorv8 wrote:
Land Rover do not recommend reboring the block. This is why there are no genuine parts oversize pistons.
I remember seeing a Volkswagen VR6 engine in bits and pieces at local engine shop a few years back. It was worn and needed a rebore but there were no OEM or aftermarket oversize pistons available. Maybe factories calculate that people buy new cars after a few years so there is simply no need for engine rebuild especially if the body itself slowly corrodes away. Basically hte engine lasts the lifetime of the car itself.

yeah that would sound about right. The 4.6 engine I got had covered 170,000kms and was bascially worn out. The RR that it came out of had so many faults that it was not worth repairing, so fair to say that vehicle had reached its "use by" date. By comparison, the 2001 Mondeo 2.0 that I sold the other day had just clicked over 300,000kms, and was going as strong as the day I bought it 8 years ago.

cheers Conrad

ChrisJC · Post by **ChrisJC** » Tue Apr 30, 2013 1:03 pm

It is worth bearing in mind that the liners are already pretty thin in the 4.6 engines, Land Rover made them as thin as they could get away with.

Chris.

DEVONMAN · Post by **DEVONMAN** » Tue Apr 30, 2013 4:36 pm

ChrisJC wrote:It is worth bearing in mind that the liners are already pretty thin in the 4.6 engines, Land Rover made them as thin as they could get away with.

Chris.

Are the liners thinner on a 4.6 compared to a 3.9 block?

I have rebored 3 no 3.9's to +20 thou without problem.

Regards Denis

topcatcustom · Post by **topcatcustom** » Tue Apr 30, 2013 5:25 pm

I'd think it is possible- as the later 4.0+4.6's are more prone to liner cracking than the 3.9's- maybe it is down to thinner/poorer quality liners?

bigaldart · Post by **bigaldart** » Tue Apr 30, 2013 6:36 pm

The liners are the same, the cracking is a result of the core shift when the block is cast, the larger journal blocks are more prone to core shift for whatever region, we run Chevy 305 pistons at 36 thou overbore so +20 should be no problem. The iron liner is not the weak point. It's the aluminium behind it.

Alan

Dixie6789 · Post by **Dixie6789** » Mon Dec 02, 2013 8:34 pm

bigaldart wrote:The liners are the same, the cracking is a result of the core shift when the block is cast, the larger journal blocks are more prone to core shift for whatever region, we run Chevy 305 pistons at 36 thou overbore so +20 should be no problem. The iron liner is not the weak point. It's the aluminium behind it.

Alan

Hi Alan.

Are you using Chevvy 305 pistons on a std 4.6 crank?

thanks Mark