Author Topic: CB750 K1 crank bearings, plastigauge and thoughts  (Read 1378 times)

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Offline Ace

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CB750 K1 crank bearings, plastigauge and thoughts
« on: April 03, 2011, 04:14:20 am »
G'day All,

I've finally stripped the K1 engine right down.  Glad I did, 3 years of metal floating round didn't do the main bearings any good.  Problem cam from the rubber cam chain guide wheel, it was stuffed so the cam chain slapped against the sides of the barrel in the tunnel.  Anyway, this is the first time I've done the plastigauge thing and I need this engine back together sooner rather than later.  25,000 miles on the engine since last full rebuild. So here goes my thoughts:
Crank case code - BBBBB
Crank shaft code - ABCAA
Bearing shells in engine -      Green  Green Brown Green Green
Plastigauge measurements - 0.038  0.042  0.050  0.050  0.050

Shells have score makes and a grove on some shells I reckon from the aluminium running through the oil.  If you run your finger nail over the crank shaft journal, you can feel that it isn't smooth and corresponds to the shells with marks etc.

Can I use the same colour (new shells) or should I change?  Will new shells of same colour reduce the clearance as they are new not worn so the plastigauge will be squished out more like #1 reading above, tighter fit, less clearance.  Bike won't spend much time at red line (I hope), port & polish with good midrange cam, JE 836cc kit.  When I ride I enjoy riding it hard.  I've read Hondaman's book and a few of his posts on this issue, I'm just after some expert out there who could lend a hand.  If it was your engine, putting lots of your hard earned cash into a classic bike to live forever and ride it not just sit on it, what would be the best option apart from paying someone else to do it.

Hopefully I've provided you with all the information you need.

1971 CB750 K1
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline bryanj

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Re: CB750 K1 crank bearings, plastigauge and thoughts
« Reply #1 on: April 03, 2011, 05:48:30 am »
No piont plastigauging old shells as you have no idea how worn they are, you need to repeat the exersise with new shaells to get a relevant reading.

I would normally say that any feelable wear on the crank journals means that they are toast and a new/good replacement crank is needed as they cannot be re-ground
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!