Big end bolts different sizes

[scherts]

I am overhauling a 3,5 and bought new bolts for the main bearings. Genuine LR part 602130 as indicated in the P5b parts list. However, these bolts are different from the original bolts. Although they have more thread the effective thread is shorter as these bolts are 2,5 mm shorter. In order to avoid stripping the thread in the block at any case, is it better to use the original ones or the new shorter ones? I know ARP bolts are the safest bet, and I will use them on the heads, but I feel buying them for the big end is a bit like overdoing this job.

I would like to include a picture to demonstrate but I don’t know how to do this

[enigmas]

Are you building an engine intended for racing or has the engine been put through severe duty cycles that you believe has compromised the integrity of the main bearing bolts?

If you really feel the need to replace the main bearing bolts use studs rather than bolts as these fasteners are less likely to compromise the threads in the block once fitted correctly. They also have the advantage of having a finer thread pitch for the cap securing nut.

Having said that, the original bolts aren't "stretch to yield", so generally can be used again. If there's a failure when torquing the bolt to its correct loading, it's generally down to a pulled thread in the block due to over-torquing, galling and unwanted friction at the thread or head of the bolt.

It's good practice to always clean the thread in the block with a suitable pristine thread tap (this will remove burrs and grit, etc.) Check the seating surfaces of the cap where the bolt head will seat to ensure that it is flat and not cupped or gouged. If not, ensure that it is. When fitting the bolts "very lightly" oil the thread and heads so that friction during the torquing process is minimised. You don't want to hydraulic the bolt in the threaded hole.

If you carefully inspect the OEM existing bolts you can't go too far wrong.
Conrod bolts are usually under the most stress due to the combined reciprocating and rotating function of these components in operation. So check them out carefully.

I've never had issues with rebuilding both Rover and P76 V8s and have reused the existing fasteners but do not stress the engines to the max. A race or severe duty engine is a different story.

Yes you can replace every component for piece of mind but attention to detail is more relevant. A compromised fastener will generally let you know as it will fail under torque if you are attentive to what you are doing.

A primary check for my builds is the conrods. Do have the conrods resized as these will have become oval during engine's life span.

[scherts]

Thanks for your advice. I was advised not to reuse the original bolts and I’m a bit surprised new genuine bolts are not the same spec. I guess I will reuse the original ones which are pristine. And no I’m not building a racing block but rebuilding for my p5. The original block had slipped liners and lower compression pistons and I’m moving to a late p6b high compression block which should also be suitable and efficient for LPG. I understood conrod bolts are very tough and rarely need replacing. They look good and caps can be removed without too much effort.