Author Topic: A7 big ends tight, what am I doing wrong  (Read 525 times)

Offline Robthebluesguitarist

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A7 big ends tight, what am I doing wrong
« on: 01.06. 2016 17:00 »
Rebuilding bottom end of A7 shooting star engine, on doing up the big ends, con rods are tight
Have used new shells -10 same as the ones I took out
Am I doing something wrong
Advice eagerly required
Near Cambridge UK

Offline coater87

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Re: A7 big ends tight, what am I doing wrong
« Reply #1 on: 01.06. 2016 17:17 »
 You need some plisti-gage. I dont know if its common or even named that over there, but its a plastic thread-like material you can crush down on your rod journals and measure clearances.

 I once had the same thing happen on a V-8 engine, one rod was completely tight with new bearings. Long story short they were all marked +20, but one pair was really more like +15-16. The parts guy thought I was nuts until he measured the one set himself. That's when I learned about and started using the stuff.
Central Wisconsin in the U.S.

Offline jachenbach

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Re: A7 big ends tight, what am I doing wrong
« Reply #2 on: 01.06. 2016 17:19 »
I assume you've oiled them before installing. Best to put the shells in the rod and cap, then smear a light coat of oil on the crank journal. Oil between the bearing shell and rod or cap will cause them to be tight. Sounds silly I know, but I've had it happen.

Offline Robthebluesguitarist

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Re: A7 big ends tight, what am I doing wrong
« Reply #3 on: 01.06. 2016 17:26 »
Thanks guys,  I may well have smothered them in to much oil, given up for the night, will  clean them down and try again tomorrow.
Near Cambridge UK

Offline cableguide

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Re: A7 big ends tight, what am I doing wrong
« Reply #4 on: 05.06. 2016 20:18 »
New pattern shells or NOS?
Check no grit between rod and shell..be scrupulous only takes a tiny bit to throw it out of whack. Then check journals for round...had stuff come back to me that was all over the place...

Online muskrat

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Re: A7 big ends tight, what am I doing wrong
« Reply #5 on: 05.06. 2016 22:03 »
Another point is that the rods will tighten up a bit until the correct torque is applied to the nuts. The rod & cap have a small crush on the slippers.
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'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Muskys Plunger A7

Offline Robthebluesguitarist

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Re: A7 big ends tight, what am I doing wrong
« Reply #6 on: 09.06. 2016 20:58 »
All sorted. Yes NOS, but Muskrat was right, when torqued up they snapped then totally free.
How many of these have I done in the past when I was a lot (lot) younger and never had a problem.
Bottom end now built and ready to pop back in the frame, providing my back doesn't pop doing It!
Thanks guys, much appreciated.
Near Cambridge UK