Main crank strap / cap help
Turbo Mini Forums

Hi, so I’m new here but not new to mini’s or building engines but never done a turbo engine before, so after some advise of the main crank cap, I have a turbo engine block with new 15cc pistons ect and hopefully by the end of the build my aim is for 140 to 170 BHP (fingers crossed) but as I’m looking for a high figure from a a-series engine what is the best / strongest option for the main crank cap ? I have a bog standard one at the moment witch I know will need a good quality strap adding to it or would it be a better option to add a new set of caps but the middle being upgraded to a 4 bolt system instead ? I know 4 bolt caps are mainly for high rev engines but was wondering if turbo engines benefit from this because of the torque increase ? I know lots of works is need for this because of online boring ect but if it makes the engine stronger I’m happy to spend the money or is this no better than having a strap fitted ?

Any help would be great, many thanks

Hi and welcome,

There are cars on this forum with 200+ Hp that use std caps and one of MED's centre caps.
Replacing the bolts is good practice, I'd (after a bad experience) also recommend having your block line honed or at least checked.

Shane

Is there any strap people prefer ? I was looking at the MED one personal also what are the best bolts to buy again was looking at MED

I used a cheap one until my last rebuild then went over to the MED one on recommendation of a local specialist and it is good quality.
As for bolts, ARPs?
I may be wrong but thought I saw a replacement set on MED's site recently?

Shane

https://www.calverst.com/technical-info/eng...ain-cap-issues/

HOSS

I look at all the evidence, and decided to just use ARP studs, nut and washers. I also used the uprated dowels that MED sell.

I understand it is the cap moving around which causes issues.

Don who used to be the Don in Aldon checked the bore was oval.

HOSS

Bog standard is fine for that power, as are the bolts.

Strangely years ago I tried fitting ARP studs....except then the gearbox wouldn't even bolt on.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

What sort of BHP is the main strap able to take ? As I said I’m totally new to turbo engines, the parts I have been buying so far I am told I could get 200bhp but thought I would aim for 140-170 to be safe but if I decide to turn boost up or upgrade the turbo then maybe I should make the crank strap able to cope with 200bhp, so I know i know I’m being abit vage but I still buying the last part for this build and want to get it as right as I can first time around. I have a phase 2 cam and from what I understand they get power up to 6500rpm if I’m correct (feel free to correct me)

I am curious about this question also. I have machining and head work done by one of the best A series builder in AUS and he believes totally unnecessary even for high HP unless your continually in the high rpm range such as racing.

https://russellengineering.com.au/services/

The ARP stud / nuts only fit the centre carrier.

Won't clear the gearbox on the outers.

Must be designed for sprite / moggie in line applications.

The standard bolts are good. They are place bolts.

https://www.bluetoad.com/publication/?i=388...owser&ver=html5

HOSS

I have never seen a picture of a broken standard centre main bearing cap. Has anyone got such a picture?

I have seen them slightly oval when checking bearing crush during a rebuild.

I have also seen evidence of them moving around in use - with a figure of 8 shape marked between the carrier and the block.

HOSS

I think it is not about BHP more about how high reving the engine is.

Have a read of the Calver article I posted a link to.

A badly fitted strap with crap bolts and washers is far worse than a standard bottom end.

HOSS

thanks for the link was a good read i think my engine builder was spot on then. i like the arp washer suggestion and swift tune cap dowels

Yes I have to agree the link is a very good read very interesting and makes sense, so worth taking in to account. I did research before posting on here and get quite a bit of feed back from the MED main cap and MED also do a YouTube video on the reason behind the cap and what it does witch also makes perfect sense, the main strap has the two steps in it so a sharp 90 degree edges each side of the bolts and this is technically the weak point and stress build up so if it is going to fail this would be the point it fails at, so by machining this flat removes the stress point but then a cap sort of replaces the area witch has been removed making the strap stiffer again not so much stronger.

But the one thing that means to stand out is it needs to be high quality bolts that are settled in properly or what ever you do is pointless without this being done correctly

Yet to see a standard main cap that has broken from a stress riser from that step.

Some one must have an example?

HOSS

Its Rpm that stresses things like rods, bolts etc, I would say leave the caps alone unless your consistenly going over 8k, and at that point go for a set of steel caps with a 4 bolt centre.

also on the topic of swapping the caps, obviosly you will need a line bore, which in turn screws with the trasnfer gear mesh slighlty, I thonk some places skim the whole bottom of the block to stop this, but some dont. also Ive seen steel main caps that are just line bored, and not honed after, so the finish is not great, according to the gread Smokey Yunnick, a hone makes a signoficant difference in getting heat out of the bearing and into the block via increased mating area.

Generally as turbo motors make power via lager cylinder pressure, the caps/ rods etc are not loaded in the same way, and that load is far less than the rpm stress. Your better off putting the money into better pistons, oi jets, head work, and of course setup. the meain thing i recomend upgrading, is ( apart from the cap dowels) fitting arp rod bolts because the originials will have been in there for years, with unknown history,

Hi thanks for the reply’s they have all been super helpful and how gives me a better idea of the route to go down with the engine.

Also if anyone is interested I have done more research in to why the main strap cap was originally used, as people have been mentioned where are the pictures of failed main straps ect well yes the step in the strap is a stress point but the stress to crack it needs to be really high hence why you can’t find any pictures of failed ones, but it looks like the cap came about more to help with crank whip not to add strength to the main cap, I’m not 100% shore how this helps control the crank whip so won’t put it up until I get a better understanding but all looks like it’s aids in the high rev down shifts in mainly race cars.

I recently have had a block line bored/honed twice (long storey) by two different expert firms, neither have mentioned the drop gear mesh post honing.

I think its largly overlooked, as it will still work, but back lash will change, it jsut depends how much by. on thing that ccan work to restore the backlash is to omit the gaskets between the block and box.

Hi Joe whats this oi jets you mention?

Its been practice for a number of years to install oil jets that squirt oil at the under crown of the piston giving both cooling to the piston under crown and lubrication to the little end.
Its well documented on here, a little abuse of the search button should dig up the info.
Common jets used have been zebec and BMW.

Shane

Slater sorted the machining for my oil jets.

I used BMW mini jets with the ball valve in them.

HOSS