4.2 pistons

paul c · Post by **paul c** » Mon Oct 12, 2009 9:47 pm

Can anyone advise the compression height of 4.2 and 3.9 pistons, are they the same, I am trying to turn my 3.9 serpentine into a 4.2 and was hoping a set of rods and crank were the only parts I need to find.
Cheers.

CastleMGBV8 · Post by **CastleMGBV8** » Mon Oct 12, 2009 10:53 pm

Paul,

4.2 pistons have a different comp height to any other rover piston and as far as I know are not available in oversizes.

You could as I did and fit the 4.2 77mm crank, a set of chevy style rods from V8 Tuner and some Keith Black Hypereutectic pistons from Summit,this give a capacity of 4350cc.

If using 36cc. heads you will want a piston with approx 13cc. bowl.

If interested can supply more info.

The rover block has a height of 8.96" ie centre of crank bore to top of decks so the critical dimensions are the throw of the crank(half the stroke) + rod length + compression height. to bring the pistons to say within 10thou of the top of the bore.

JE has 4.2 cranks for £260, Rods £300 pistons from summit shipped approx £300

The real steel 5.0 stroker Kit with forged pistons for approx £1600 also looks good value.

Just noticed your location so other sources for crank may be cheaper.

Kevin.

paul c · Post by **paul c** » Tue Oct 13, 2009 12:32 am

Thanks Kevin, I will have to get the heads off and measure the block, but when I replaced the head gaskets and head bolts it appeared to be like new, it has no stamping so I think it may have been a factory replacement block, I may get away with standard pistons. Whoever assembled the engine last had fitted tin gaskets and left the head bolts loose, it has 10 bolt heads. Though finding a factory crank and rods may be difficult too.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 13, 2009 1:00 am

Paul,

As you have 10 bolt heads, does the block have cross bolted mains as wondering if it might be a 4.0. If so a 4.6 crank and rods will drop in and with 4.0 pistons give approx 10.8/1 compression and with big valve heads will give 250 + BHP and loads of torque which is probably looking for.

Kevin

paul c · Post by **paul c** » Tue Oct 13, 2009 1:11 am

Hi Kevin, no the castings are there for the cross bolts but not drilled.
I have a 4.0 here but it has been hot and the top of a piston is broken, so the chances of the block being good are slim, as a 4.6 shortblock is easy to find, but the same block issues. I suspect a 4.2 would be more economical than a 4.6, the 3.9 is surprisingly good on fuel but struggles a little towing on hills.
I bought the 4.0 for the thor manifold, which I intend to modify for the hotwire injection. Maybe I should just do that first and see how it performs then, I have heard very favorable comments about this conversion, almost 30% torque increase.

Wotland · Post by **Wotland** » Tue Oct 13, 2009 11:17 am

Hi Paul,

for pistons you can use low comp 3.9 pistons sligthly machined.

They are very cheap. V8Tuner sells them in +0.020" for £140.00 the set.

paul c · Post by **paul c** » Tue Oct 13, 2009 11:45 am

Thanks Dimitri, I was going to ask about that as I see there are some for sale on Ebay, they are selling them modified ready for a 4.2.

I will try getting in touch with JE about the crank, at those prices it would be worth doing.

Did you get in touch with John Harcourt?

spend · Post by **spend** » Tue Oct 13, 2009 12:18 pm

At those prices + shipping, have you considered grinding down a later crank? Aren't some of the 'odd' 3.9's large bearing anyway?

paul c · Post by **paul c** » Tue Oct 13, 2009 7:56 pm

spend wrote:At those prices + shipping, have you considered grinding down a later crank? Aren't some of the 'odd' 3.9's large bearing anyway?

Might be worth looking into prices, by my calculations I should be able to get a 4.6 crank down to stock 4.2 sizes, is there any difference in the counterweights on a 4.6 crank ?

RoverP6B · Post by **RoverP6B** » Tue Oct 13, 2009 8:04 pm

Hello Paul,

Even after turning the journals down, the 4.6 crank won't fit into a 3.9 block as the rotating radius of the crankshaft is too large.

Try and locate a 4.2 crankshaft if you can.

Ron.

paul c · Post by **paul c** » Tue Oct 13, 2009 10:06 pm

Thanks Ron, I have been offered one, polished and checked for 50GBP, plus shipping, so think that will be the go, then a set of 3.9 low comp pistons.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 13, 2009 10:31 pm

Paul,

Sounds a good deal and should produce a useful increase in torque and if you can match the new piston weight to the old 3.9 ones you can probably get away with not havaing a total re-balance.

Kevin.

paul c · Post by **paul c** » Wed Oct 14, 2009 1:41 am

Wouldnt they need to be the same weight as stock 4.2 pistons, as that would be what the crank is balanced for?
Will probably balance the lot anyway, make things last a bit longer and to be safe, but the closer it is to start with the better.

Hopefully it will give a good increase, having a longer rod than the 4.6 should help too, I was very happy with my 351w when I used the longer 302c rods in it.

Wotland · Post by **Wotland** » Wed Oct 14, 2009 8:45 am

Hi Paul,

unfortunately no success with Harcourt

.

If ever you don't buy the 4.2 crank let it me know as I have an friend who is also looking for an 4.2 crank.

Kevin did you need to machine the top of your chevy 305 pistons ? If yes how much did you pay to machine them ?

CastleMGBV8 · Post by **CastleMGBV8** » Wed Oct 14, 2009 8:50 am

Paul,

You are correct

the pistons will need to be the same weight as 4.2 items, if the modded low comp 3.9 pistons work out lighter you will need to remove some weight from the crank counter balance weights.

Cant remember so worth checking if the balance of the 3.9 and 4.2 was external with additional weight on the front pulley to balance out the different internal weights if that make sense, if you are building a low RPM high torque engine this may not be too critical.

Kevin.