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Author Topic: Rod bearing clearance  (Read 4153 times)

Offline Dunc the Funk

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Rod bearing clearance
« on: May 23, 2020, 02:39:46 AM »
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.
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Offline masracingtd1167

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Re: Rod bearing clearance
« Reply #1 on: May 23, 2020, 04:51:10 AM »
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 !
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Offline Dunc the Funk

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Re: Rod bearing clearance
« Reply #2 on: May 23, 2020, 07:07:34 AM »
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.
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Offline wideopen231

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Re: Rod bearing clearance
« Reply #3 on: May 23, 2020, 07:22:25 AM »
IMO its better to error with .001 or so too much rather than too little.  Aluminum or steel rods?
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Offline Dunc the Funk

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Re: Rod bearing clearance
« Reply #4 on: May 23, 2020, 09:17:47 AM »
Aluminum (MGP).
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Offline Paul New

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Re: Rod bearing clearance
« Reply #5 on: May 23, 2020, 06:47:18 PM »
I can tell you with my GRP rods I run Clevite CB663-HND in my combo and run .0035 clearance granted when I ordered my crank I ordered it sized wheee I wanted it.
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Offline THE FRENCHTOWN FLYER

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Re: Rod bearing clearance
« Reply #6 on: May 25, 2020, 07:34:19 AM »
Quote from: Dunc the Funk on May 23, 2020, 07:07:34 AM
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.

My crankshaft grinder does not trust Plastigage readings to give an accurate clearance measurement. Use a dial bore gauge.
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dreracecar

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Re: Rod bearing clearance
« Reply #7 on: May 25, 2020, 08:27:54 AM »
Unless you cycle the rods constantly. you should run alu rods a couple times and then pull them for re-sizing anyway to straighten out the roundness in the bores after they take their first set-- doing this increases the lifespan on everything and get an accurate measurment
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