One of the benefits of using a mk2.5 block over a mk1 or mk2 is that it comes with a Main Bearing Support Plate (MSBP). Essentially this is an uprated windage tray that sits between the sump and the block. Here it is:

So, rather than just bolt at the edges, this one also bolts into the main bearing caps (4 of them anyway). This is supposed to reduce flex at high rpms. I suppose, given that this engine will rev higher than the mk1, this is a good thing.

However, it won’t fit a mk1 or mk2 sump. Here’s my mk1 sump… and I apologise for the mess. It looks like when it was fitted most of the world’s supply of RTV sealant was also used. I’d scraped it off to help the photos but not cleared it from the inside of the sump yet.

This has got the cut outs for the standard windage tray to sit into. The standard windage tray only occupies 3/4 of the length of the sump, where as the MBSP goes all around it to the moon gaskets. So, if I were to fit it like that, the sump would be the thickness of the MBSP plate too low, and there’d be a gap!

Of course, there’s another problem too. The ‘dimples’ for the crank counterbalances don’t fit inside a mk1 sump.

So, I’m left with a decision. Park the MSBP on a shelf and use the normal windage tray, or modify the windage tray and sump to fit. Of course, this isn’t a problem for the MX5 guys as they could just use the mk2.5 sump. However, I’d need to modify that and the guy that did my sump has already said he won’t be doing another.

Modifications required to my sump would essentially be machining the face of the sump down by the thickness of the MBSP. I’d then need to cut out the dimples on the MBSP itself.

Fortunately, it’s not a decision I have to make right away, but I suspect I’ll need to have made it in the next couple of weeks.