Crank tunnel sizing

J

jimmmy57

Member
Hi Grumpy,
Thanks for this site and taking the time to look at this for me
Am building a 383 at the moment using a gm 2 bolt block that had 4 bolt caps installed by previous owner.
The job they did fitting the caps appears ok and it looks like the tunnels were line bored.

Problem is the crank main bearing clearances are tight and I think maybe the cause is the tunnel sizing is on the small side
and they're not very uniform.
I have found this acl site that specs tunnel tolerances http://www.aclperformance.com.au/us/Che ... ingsus.htm.
Do you think if I have the block line honed it will sort the main bearing clearances out?
What about bearing crush? i guess the crank tunnel size dictates this.


Here are the measurements I have.
Journal Block Main Bearings Crank

No 1 2.639.2" 2.448.8 2.448.3
No 2 2.638.5" 2.448.9 2.448.3
No 3 2.639.4" 2.448.6 2.448.0
No 4 2.639.6" 2.448.8 2.448.2
No 5 2.639.6" 2.449.0 2.447.7

I think this is a good example to everyone why everything needs to be checked
 
ok, first let me say,.SORRY it took a bit of time to reply because IM in the car currently, and need to dictate to one of my sons. so I can,t look up the correct dimensions currently.
your 100% correct that you always need to verify clearances, and its great that you took the effort here rather than slapping components together and being amazed when things fail later "FOR NO OBVIOUS REASON"

ok, your local machine shop can advise you on which route to take and the cost associated with each option, obviously uniform main bearing bore sizes, and concentric bore axis are ideal, but thats rarely found, cranks are rarely uniform either although they tend to be fairly consistant if new and never pollished/cut.
your variation in dimensions doesn,t look like its all that uncommon, from what Ive seen in the past, but those dimensions look wrong.
the obvious goal is to provide the correct alignment bearing crush and bearing to crank clearance, oil acting like a coolant flow between the bearing and crank, to maintain the correct lubrication and low stress
the main bearing saddles must be close to correctly alighed or the crank will be constantly under stress and the bearings will soon wear incorrectly resulting in loss of the correct support and oil pressure dropping off as the clearances slowly increase on the end bearings.
obviously your aware that main bearings come in .001, .002 under and over sizes also, and although having the block honed to size is ideal its fully possiable to mix & match bearing under/over sizes to compensate for minor differances, in both main cap,crank tunnel bore sizes and crank size variations, PROVIDED both the crank SADDLE & BORE and CRANKSHAFT are STRAIT.AND VERY CLOSE TO THE CORRECT SIZE, IF they are NOT strait and concentric to the same center honeing the bore is almost mandatory unless the variation in centers is EXTREMELY small.
Id suggest asking your local machinest what they suggest, ID be inclined to get the block align honed and carefully checked before proceding, get it consistant,and correct, but that costs a good deal and in your case the use of slightly differant bearing sizes might provide the correct bearing to crank clearances.but Id sure verify that with the local machine shop after verifying the dimensions carefully.
the reason I suggest verifying is you won,t be the first guy to meassure incorrectly, and having the local shop verify and if necessary correct the main bearing saddle hone job, may be mandatory
I try never to B.S. people so
I sent a PM to CNC BLOCKS for thier input here as well.
1-207-892-7215
http://www.cncblocksnortheast.com/


viewtopic.php?f=51&t=831

viewtopic.php?f=51&t=49

viewtopic.php?f=53&t=88

viewtopic.php?f=51&t=134
 
jimmmy57 said:
Hi Grumpy,
Thanks for this site and taking the time to look at this for me
Am building a 383 at the moment using a gm 2 bolt block that had 4 bolt caps installed by previous owner.
The job they did fitting the caps appears ok and it looks like the tunnels were line bored.

Problem is the crank main bearing clearances are tight and I think maybe the cause is the tunnel sizing is on the small side
and they're not very uniform.
I have found this acl site that specs tunnel tolerances http://www.aclperformance.com.au/us/Che ... ingsus.htm.
Do you think if I have the block line honed it will sort the main bearing clearances out?
What about bearing crush? i guess the crank tunnel size dictates this.


Here are the measurements I have.
Journal Block Main Bearings Crank

No 1 2.639.2" 2.448.8 2.448.3
No 2 2.638.5" 2.448.9 2.448.3
No 3 2.639.4" 2.448.6 2.448.0
No 4 2.639.6" 2.448.8 2.448.2
No 5 2.639.6" 2.449.0 2.447.7

I think this is a good example to everyone why everything needs to be checked
Click to expand...

I can see there is a big problem here as we sell all our blocks with the mains line on the high side which is 2.6415 and some books its 2.6416.

And we alwyas ball mic our bearings for shell thickness asI have build sheets from some of the cup teams and this is there method as well, On most std. main beaing sets the shell thickness is .0955

Number one journal is 2.4483
Housing bore sould be 2.6415
subtract the two .1932
Shell thickness X-2 .1910
Differance is .0022 is your oil clearnace.

We also checked this against the Sunnen bore gauges and have come out with the same numbers, And its less time consuming measuring the shell thickness.

And we have been using this method for 37 years no with zero problems with bearings.

I would say the block was never finished with a line hone and the shop should give you a torque spec and sequence of how they torqued the main bolts.

Every block we ship has those specs with it as we have seen it does make a differance on how the caps are torqued up.

All performance block work should be finished with a line hone as we have seen that line honing sure brings out the imperfections of line boring.

And ? give me a call 207-892-7215
 
Thanks so much for your quick and informative replies guys !!!
I was getting bogged down on it.
Thanks for the offer of calling

I'll keep you posted with the outcome

Cheers, Jimmy.
Melbourne. Australia
 
OK,
spoke to the machine shop today and they're worried that the caps are shifted 0.003' sideways.
They say it will leave an area near the parting line that the back of the shell won't have contact with the tunnel (would this be ok, what is alowable), or they remachine the caps and start all over again costing a lot more.

So,,,,, they didn't spend anymore time on the job and want me to get another block to bore as if it's not totally nessesary that I use 4 bolt caps (although it would be nice), will probably be about the same price but nicer .

I know you guys don't know exactly what I'm dealing with , but would appreciate some general "if it was mine " advice

Cheers, Jimmy
 
machining the current blocks an option but from what you posted it doesn,t seem the current machine shop thinks its a reasonable route to go, since THEY have inspected the block ID suggest taking thier advice!
remember the stock castings were never ment to exceed about 400hp, adding 4 bolt caps helps but your still working with a thin casting, and yes IM fully aware that those castings are frequently pushed too over 600hp , IM also fully aware of the failure rates, if they are constantly subjected to the stress, keep in mind having an engine that made 450,or 500,or 600 hp on a dyno test, once for bragging rights and gets run once or twice a year at the track, but that spends most of its life lopeing at idle or cruising the local hot rod crowds, at car shows is hardly under the same stress as an engine running the car at high power levels racing, every week,
theres two basic routes to take, in my opinion,SWAPPING too the stock two bolt block route, with ARP main cap studs will be just fine if your looking to make under 450hp., and stay under 396 displacement, cheap, easy, little problems. but little room for real expansion.
if your seriously thinking of going over 500 hp,or beating on the engine frequently, a true thick wall casting thats easily three times as strong, and with the added benefit of having a block that can easily support 427-434 cubic inches.. theres really little realistic choice other than a DART BLOCK

http://store.summitracing.com/partdetai ... toview=sku

viewtopic.php?f=51&t=47

having gone both routes all I can say, is if the budget allows the DART blocks the route to take, and yes IM broke most of the time also. but in the long run the DART block cost less as its far less prone to problems
 
Grumpy and Carl,
The outcome of this excerise has been that the guy has refunded my money and the short motor returned.

Thanks so much for your opinions and advise as it has helped me a lot with my desisions.

I am considering a rebuilder block and new rotating assembly with my top end.
No one here has them in stock except this guy who has Procomp assemblies, what do you think?

http://cgi.ebay.com.au/NEW-CHEV-350-383 ... 7C294%3A50

i am having the T700 being built atm and will probably fit this to the car with the current 350

Thanks again. Jimmy
 
that (LOOKS LIKE) a bargain on the base line kit parts,
(YOU DID NOTICE THE BLOCKS NOT INCLUDED DIDN,T YOU!)
but unfortunately only close inspection will tell you the truth, Id specify the 2 piece rear seal crank design parts as matching components will be far easier, ID ask if they have a piston upgrade option to FORGED simply because the difference in cost is minor compared to upgrading later and there's a major strength difference

compare prices and components

http://ohiocrank.com/chevsb_rotate.html

http://www.adperformance.com/index.php? ... abda05a964
 
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