Forum » Practical Engine Building » Main cap clearance

Main cap clearance

Practical Engine Building

Discussion and questions related to the course Practical Engine Building

= Resolved threads

Page 1

Hey guys,

I currently have my engine down at a really reputable engine shop. Builds some of the fastest rally car engines in the world.

now they have come back to me and said that my main caps on my GTiR SR20DET are not interference Fit. Seems like it is a factory fault as the engine has never been apart and there is no sign of fretting.

It’s getting arp main studs and only putting out 350-400hp. Although it is a race car engine so will see some decent abuse.

what would be your recommendations? They have asked if I can find another block (So hard to do) or dowel the block with the main caps. He mentioned that they could assembled it how it is and it might be fine but in the chance it’s not the main caps may moved and cause bearing failure of course.

An opportunity?

With these being steel (assuming standard is cast iron) they could tig welded and remachined if still loose.

Hard to say just how significant it is - but as they're experienced, they should know.

Part of the problem is in what they call an 'interference' fit, as with non-interference (clearance) there is a range, or class, of it.

Normally, it is for alignment of the tunnel bore halves with it being a light slip fit but it can be a light interference fit with a LITTLE force being required. personally, don't think I'd be concerned for a lower level engine - but then I don't have access to the parts to make a definitive call.

As Denis pointed out, there are also steel replacements available - if you have your machine shop carefully measure the cap length and the block's recess length, perhaps one of these aftermarket suppliers can do an 'oversize' for you? It'll still need to be align bored, so the total costs will add up.

Thank you everyone for you replies. I talked to the builder and we have gone with Doweling the block and main caps. By the time I fitted oversize billet mains and got them all machined. It would have been more than just getting Dowels put in.

they CNC dowel it all so it will be a pretty solid setup.

as for the clearance question. It was definitely a clearance fit but I’m not sure by how much. I wouldn’t imagine much at all. These guys are guns at what they do so I trust in the guy.

Thank you once again for your replies, as always they’re a massive help!