Bearing Spacers

[Twentyover]

In the mid-70’s some people were putting 350 cranks on 400 Chevys to get a 377, bigger inches with better bore stroke ratio, This was in the days when the only people that made blocks were guys like Pink and Donovan, so you were pretty much stuck with stock SBC blocks. Since the 400 used a larger main than the 350, bearing spacers were used to take up the gap between the larger 400 main bearing saddles and the smaller 350 main journals.
I’m in a similar situation with an old MG engine I’m working with. There are no quality high load bearings available. While it looks like I have a rod bearing solution (early small journal SBC bearings), there are no quality main bearings available that are close. The fast guys in the UK running this engine pound out the mains about every 4-6 weekends, watching their oil pressure gauges, changing mains out when the oil pressure starts to drop away.

Nominal main journal diameter is 2.3745” and main bearing bore is 2.5212, design bearing length is 0.947 for 5 of the 7 mains, center and rear are 1.065 long. Thrust is taken up with 4 crescent shape bearings on each side of the bearing saddle and cap. Found some bearings that are close, but none of these are without issue, specifically to get the bearing bore down to the value it needs to be would take a pretty huge cut on the main caps and I’d also need to cut the main saddles. So my thinking turned to using a good tri-metal bearing for a different application, with a bearing spacer, like they did in the long forgotten past (except to us guys with lots of gray hair, if we still have hair).

Does anyone know who can make a set of custom bearing spacers? Any known (to people that aren’t me) problems with this type of plan?

I’ve got a good machinist, but I’m thinking this is kind of more specialized. Any experience with making them?

Bottom line is…..

Help!! Mr Wizard!!! (another arcane reference those my age will understand)

[1BADBET]

You didn't say what MG engine this is...... the only problem with doing a custom bearing spacer is going to be price, it's a ton of labor and it's not a cheap endeavor. Can you tell me how much power and what kind of rpm your turning? Just curious about the engine specs that would hammer mains out of the engine. I've built small chevy engines with 'P' bearings that made good power and never had a problem. What kind of oil pressure?

[Twentyover]

Engine is a 2.9l seven main bearing 'C' series engine, derived from the 4 main 'C' series used in the Austin Healey 100-6 and 3000.

Bore: 3.28
Stroke: 3.5

The engine was only used on MGC's (8998 produced) and Austin 3 liter sedans (about another 10,000 produced), so the engine is uncommon, but not particularly rare. I've picked up a couple spares watching ebay over the last couple years

Nothin', and I mean NUTHIN' on this engine is cheap. Developing it even for vintage racing is causing significant internal redesign. with a deck height of 10.25", 3.5" stroke, and 6.6" rods. Piston compression height is about 1.9"; rods weigh 1100 grams each; flywheel was built for a cement truck. Going to a 7.2" rods to get compression height and piston weight down, thinking we can get close to 8 lbs out of the reciprocating assembly. Another 20 from the flywheel, even with the stock diameter single pate clutch. Engine weighs like 650 lbs.

Don't want to spin more than 7000 with the stroke length being what it is. The target is to come close to the 275-290 hp the fast guys n the UK are getting. You'd think that w/ 7 mains, it would be OK, but there are only aluminum backed bearings available. Hot oil pressure is 55-60psi at rpm- there are high pressure relief valves (Oil pressure relief valve is mounted in the block) for 75 psi- I'm considering running an external wet sump pump like a Peterson to keep the distributor/oil pump drive gear from shredding- I am required to run a distributor based ignition.

Most guys are running three 40 or 45 DCOE's, I will be running 3 HS6 SU carbs.

https://static1.pt-content.com/images/pt/2017/02/80e618ac71b2c79d39b5a82a4fe68dff-1.jpg

[1BADBET]

Have you looked at bearings that fit your existing housing bore diameter and changing the crank journal by cutting or welding it (depending on what's available) narrowing bearings is easy and doing the crank would be cheaper than making a custom spacer if you could get it all to work.

[Twentyover]

Have you looked at bearings that fit your existing housing bore diameter and changing the crank journal by cutting or welding it (depending on what's available) narrowing bearings is easy and doing the crank would be cheaper than making a custom spacer if you could get it all to work.

In general, the bearings used on MG's tend to be thicker than many US applications. For example, the SBC rod bearing i'm using, for instance, is about 0.010" in thinner wall than the bearing designed for the engine. I'm having the big ends for the rods originally cut for the thinner wall SBC rod bearing, rather than the factory specified bearing. To use any bearing in the existing mains looks like it would require welding the crank

i looked into welding a crank associated with a discussion on stroking cranks. I was quoted $200 per journal, aon with another $75-100 journal to straighten the crank.

So I'm not feeling the economy of welding the main journals up

[1BADBET]

Wow that sounds way too expensive... how much does it need to be welded up and I'll talk to my crank guy see what kinda price I can get.