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MG Midget and Sprite Technical - 948 conrod ARP bolt torque
|Does anyone know the correct torque for ARP bolts for the standard 948 conrods?|
I installed them years ago, but have now dismantled the engine to correct some obvious mistakes. The engine has not been run in the meantime. Of course I have now lost the instructions that came with the bolts.
|From what I can see its listed as 35 lbft|
|Is that for the ARP bolts with the special ARP lube? Or for the standard rod bolts?|
|Oops sorry Tore answered without reading correctly - that was for standards|
|id call arp for that info... to important to risk being wrong about|
|Prop and the Blackhole Midget|
|I have sent them an email, but no reply so far.|
I found this - does it help?
|Thanks - according to this, 45 lb/ft looks about right. 10 more than the standard bolts?|
|I'm no expert on the A series and can't remember for certain the size of the bolts, but I >think> they are 3/8" - is that right?|
Also it also depends on the tensile spec of the bolts you bought.
The ARP catalog lists A series 3/8" rod bolt set as part number 206-6001 here:
Entering that number on the "instructions" page of the website brings up the page for that kit, here:
On the page is a (blue) link to the instuctions. It says the instructions open in a new tab. I tried it and I got a PDF download.
This tells you to torque the bolts to 55lb/ft, using the ARP lube, as they are the high tensile strength type.
Please don't take my word for it Tore - have a look yourself before you make your mind up. If you can't get the PDF I'll be happy to email it to you, but I can't attach it to a post on the BBS.
The site also lists bolts for BMC A (and B) series which are 11/32" diameter. (Kit number 206-6002). I speculate these are for later engines, though the site doesn't say so definitively.
+++++For the 11/32" bolts the specified torque is 35lb/ft, so the size of the bolts you have is critical.++++++
Both instruction sets specify a bolt stretch value of 0.0065" to 0.0070" if you prefer to use the bolt-stretch method of setting the clamping force.
Interestingly they say the conrods should always be re-sized after new rod bolts are installed. Can anyone tell me what they mean exactly?
|Thanks for all the information, Greybeard!|
The instructions are still missing, but I have found the receipt from Slark Race Engineering, where I bought the bolts! So I sent an email to them.
Neil Slark answers that the correct torque for these bolts is 45 lb/ft. The 948 rods are of course the early type with threads in the rod, no nuts are used. The correct torque for the original bolts is 35 lb/ft, and for the later type of rods with original bolts/nuts it is 45 lb ft.
10 lb/ft more than standard with these high tensile bolts seems reasonable to me. I just hope the threads in the rods can take it!
|Ah - that's good Tore. Well done!|
I'm not sure if you are asking what re-sizing is, or why you have to do it. Re-sizing is done to the large end of the rod with the cap bolted on. It is a honing process I believe, and I think the cap or rod is shaved a bit and then the hole is made round and dimensionally correct again. As to why it is done after installing new rod bolts, I can only guess that punching in the new bolts takes the rod out of round. My machinist told me it needed to be done after installing new bolts.
|C R Huff|
|Thanks Charley - that's about what I thought, but I was wondering why it was needed after installing new bolts. Anytime I've had anything to do with new rod bolts it was after a crank regrind and new shells; the new bolts were fitted at the same time for the sake of a bit of piece of mind and they were always carefully pressed in. Maybe in those circumstances resizing was a bit academic.|
Never done it on a small engine, only biggish diesels.
Thanks anyway, I was just curious.
|But on these rods the bolts are not pressed in, they are a quite loose fit in the cap, and are screwed directly into the rod. I suppose no re-sizing will be needed then?
They could be out of round from age/use, having nothing to do with new rod bolts. I think they are measured on the same machine that is used to re-size them, and it should be cheap to have them checked. Then if they need it done, it would be false economy not to do it.
|C R Huff|
This thread was discussed between 11/09/2015 and 14/09/2015
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