Technical > Matt Shaff's Engine Shop

Rod bearing clearance

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Dunc the Funk:
My local machine shop checked our crank & gave it a very light polish. The guy said the rod journals are at standard. The bearings used by last year's rebuilder were standard (Clevite CB-743HND) and the previous year's builder were one thou thinner (CB-743HXND). The rod manufacturer (MGP) says we need 3 to 3.5 thou clearance. I assumed that when we first got the engine it had been set up loose for traditional thick oil so that seemed ok. Anyway I used fully synth 10w50 the first year & 15w50 last year with no major issues.

This year I ordered King CR 808HPND in standard but before fitting I thought it would be a good idea to check clearances with last year's bearings. I only have Plastigage. Everything was dry & clean & correctly torqued. I was surprised to find clearances were between 1.5 and 2 thou. Now I could use Clevite thinner bearings as above or King equivalent (STDX type) but firstly availability is not so good and secondly 4 of the bearings will still be a little tight at around 2.5 to 2.75. My question is would it be better to have a thou or two taken off the rod journals now then I can use the bearings I bought & not have the same issue next rebuild? I'm planning on using 10w50 oil as the crank is already set for 3 thou.

masracingtd1167:
Very difficult to take .001 off of a crank and have it stay round and if the crank is nitrated it will remove that as well . I would suggest you take your rods and crank to the machine shop and have them check the clearance with a dial bore gauge ! You can change the clearance with an EX bearing and if that gives you too much clearance you can mix an EX with STD halves to get the proper clearance !

Dunc the Funk:
Thanks for that advice re machining, that's the stuff I just don't know yet. Just referred to my crank figures. After block & caps re-sized & align honed I used King X bearings on them looking for 1 thou and gained a consistent 1.5 thou on all 5 which was actually better. Probably visit machine shop next week, I'd like to know how round the rod bores are too.

wideopen231:
IMO its better to error with .001 or so too much rather than too little.  Aluminum or steel rods?

Dunc the Funk:
Aluminum (MGP).

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