Hi,

I am looking for information on Mercruiser MCM HP425's.

I have a rod blown through the sump and trying to assess whether to repower or rebuild.

I dont seem to be able to find much information on the HP 425 or where might be able to supply them.

In general its a 1993 38 topgun undergoing some major surgery as the stringer also seems to have collided with the floor.

Can anyone suggest if there is a workshop manual available for the boat or the engines?

Any advice is appreciated.

I have a manual. Ebay it.

They are basically the same as the old 420's...

Basically 454 mags with a little different cam, crane roller rockers, and Gil exhaust. They certainly aren't exotic motors. If you have a rod blown through one of them, I'd say you're much better off to start over. If the rest of the motor is in good shape you could conceivably swap over the top end onto a MAG short block and be back in action.

<<<<<<<< Do they look like these?

Thanks guys. Sorry quite new to this, usually work on things with wheels.

Looseconnection, thanks, I cant seem to find manual on ebay, are you referring to boat or engine manual? I have got mercruiser manual 16 which is close.

930Turbo, thanks for the information. Yes, they look exactly the same as the ones in the picture. Are they yours?

Starting fresh seems to be the way forward. So a 454 MAG block will have the correct mains, 7/16 rods etc? Any suggestions on where is the best place to buy?

Hopefully will post some pictures on here later.

You probably have a Gen V block, but it could be a Mark IV block too. Easy way to tell is a Mark IV has a fuel pump mount on the block, Gen V doesnt.

If you have a Gen V, then you'll want to find a gev V 454/365 to bolt your top ends too.

If your block is salvageable, you can rebuild it, using a aftermarket scat or eagle crank, or find another GM forged 4.0" stroke crank for a 1pc seal (Mark V). I have a set of Rods for your engines i can sell you cheap. Replacement for the GM pistons are avail from Speed Pro, #L2399F. Cam is available from Crane still, as they supplied Merc with the 420/425HP cam.

I think they are the same as 420, 420 had log exhaust, down and out the rear and the 425 had centre risers out the top.

The 420 never had those log manifolds. They came with Gil exhaust or a a center rise mercury high performance monster sized center rise exhaust in 1987 for the early ones.

The old 400HP cyclones did come with Rear exit log manifolds, along with the 330 and 370 HP.

Irrelevent, because if were are talking 1993 Top Gun, they should be Gen V engines, and have GIL style exhaust if its all original.

Those 420/425HP engines ran well. Throwing a rod wasn't something common, unless it was neglected or severly tired.

The 420's arent stroked motors are they?