+ .030 ID Main Bearings

[GavinT]

Gents, unbelievably frustrated with this one. Machinist messed up #1 main journal and cut it down to .025 under, was meant to go .020 under. +30 mains are available from some of the vendors but is it a good idea to go this deep?. Machine shop will correct the situation, I just need to determine the correct course of action. 

options:

1. Cut to 30 under 

2. journal repair and cut to 20 under 

2. Replace crank

This is on my 1988 910HC Turbo.

Thanks all.

 

 

[drdoom]

Hey Gavin,

Sorry to hear of that misfortune. How hard do you run, how high do you rev the Turbo? Garry Kemp advised me that while the 907 stroke crank is quite stiff the 910 does flex more. Welding the crank raises risk of micro-cracking, as far as I've seen, though perhaps your sources can do it to perfection. Where to look for a replacement crank?

Keep us posted on what you learn please.

[910Esprit]

If Lotus themselves originally listed +30 as a spare part, I wouldn't be too concerned (just a little annoyed)!   

Steve, I suspect your comments may relate to the fact that the block was stiffened for the 910 with a revised main bearing panel.   I'd be very surprised if the actual crank is in anyway 'weaker' its just that its under more load.   But that's just my supposition     

[GavinT]

Reached out to Garry Kemp. He has original OEM .030 bearings available but they are all grooved. He didn’t express concern about taking it down to .030 under.

Tim Engle also didn’t express too much concern about taking it down to .030. But added it would be beneficial to have the crank journals nitrided. His main issue was that the crank has gone as far is it can go. So any future need for crank work would result in a new crank.

[drdoom]

14 hours ago, 910Esprit said:

If Lotus themselves originally listed +30 as a spare part, I wouldn't be too concerned (just a little annoyed)!   

Steve, I suspect your comments may relate to the fact that the block was stiffened for the 910 with a revised main bearing panel.   I'd be very surprised if the actual crank is in anyway 'weaker' its just that its under more load.   But that's just my supposition     

From what I've gathered over the ages it seems quite clear that Lotus stiffened the MBP section of the bottom end for good reason, not surprising but typically purposeful. One very devoted enthusiast in California reports his exploration in tuning the 907 determined substandard bottom end reliability beyond 190BHP. The extra stroke of the 910/912 does result in flex of the crank meaningfully greater than that of the 907. This is what GK has advised me in Q & A some years ago. Further to that, one may read of the engine development programs covered in Tony Rudd's autobiography, wherein flex between #4 pin and the #5 main journal were observed in testing.

Gavin, it looks like you've got a green light to grind that crank down to minus .030. Any further insights that come your way I'd be glad to read.

Cheers

[GavinT]

Update….The issue with 0.030+ bearings is availability. The lower shells are the problem. Getting OEM quality is the next issue. SJ offer alloy 2 ply which I was advised not to installed. I emailed all the usual suppliers. LotusBits and PNM didn’t respond. SWLC didn’t have any options. Garry Kemp could offer OEM grooved lower bearings but they are not suitable for a turbo esprit. JAE are out and their supplier is not doing another run anytime soon. The winner was QED. They have both OEM and aftermarket bearings. Aftermarket in 0.030 lower bearings, but their aftermarket bearings are tri metal and are OEM spec. 
Ultimately I had a stroke of good luck and scored an extremely lightly used 910 crank that is still standard from JAE.

Shout out to Tim Engle, Garry Kemp, Dave(Changes) and Mike Sekinger for all their advice, extremely grateful for their time.