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948 Main Caps and Dowels

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jonbok Avatar
jonbok Jon Bok
Los Angeles, CA, USA   USA
I am building a 948 engine, mostly for a "spare", and to keep my mind and hands occupied.
When I took it apart, the main caps were on nice and snug. I used the slide hammer to remove them. During the process, the caps have been on and off and torqued down a few times, for Plastiguage-ing, etc. Now the caps don't seem snug on the dowels at all. They just slip right on. Should I worry? do I need to change the dowels out, and therefore line-bore the block? Put it all together and "hope for the best"?
I really wasn't really planning on going too crazy with this project.

Thanks.

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IanFife Avatar
IanFife Ian F
Inverkeithing, Fife,Scotland, UK   GBR
I'm an "amateur" engine builder (only do my own engines) so any professionals replying may have a different view to me. I've rebuilt quite a few "A" and "B" series engines fitted to MGs and other British cars, and my view is that as long as it is a nice sliding fit, and not sloppy/loose/ with any side play, it should be fine just to go ahead. Old engines tend to get a lot of varnish and can be very difficult to dismantle. Once this has gone everything should just fit as a "perfect fit" - not tight and not loose.

Just my opinion of course!

IanF.



MG Midget MK1 (1961)
MGBGT (1972)

Speedracer Avatar
Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
They are just alignment dowels, you should be fine using them, they really don't normally have much resistance to them removing, or installing them, your's was probably just stuck form being in place so long. Bottom line, nothing to worry about.



Hap Waldrop
Acme Speed Shop
864-370-3000
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dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
One of the steps often overlooked by those who do not build a lot of engines is a procedure called "setting the thrust." It goes something like this. There's always just a little bit of play in the fit of the main bearing caps to the block as they ride on their studs or nuts. One of the main caps will carry the thrust washers, and it is important that this main cap and the thrust washers remain square to the thrust surfaces on the crankshaft. The difference can be subtle, but it usually exists.

In order to perfectly square the thrust main cap to the centerline of the crankshaft, it is common to install the crankshaft, along with all the main caps, bearings and thrust washers, but only lightly snugging the main cap fasteners. Then, with a dead blow hammer or suitable implement of destruction, you whack the nose of the crankshaft rearward and then whack the back of the crankshaft forward. This will force the crankshaft thrust surfaces against the thrust washers which, in turn, force that main cap to be perpendicular to the centerline of the crankshaft. You can then torque this main cap and the others to spec, and then measure the crankshaft endplay.

Failure to perform this step can leave the thrust washers slightly askew to the thrust surfaces of the crankshaft, such that only the corners of the thrust washers are actually contacting the thrust surfaces of the crankshaft. This can lead to erroneous endplay measurements. Further, if only the corners of the thrust washers contact the thrust surfaces of the crankshaft, they'll wear quickly, and this will result in additional, and usually excessive, endplay...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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jonbok Avatar
jonbok Jon Bok
Los Angeles, CA, USA   USA
Thanks for the replies. I feel more confident now, and it looks like the engine is going to go back together very nicely.
Dick, that's a great procedure on setting the thrust washers. I would not have thought of that, and I did not see anything like that in the workshop manuals, but it makes perfect sense when you explain it. Nothing beats tapping into someone's experience.

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
thumbs up

Dick

In reply to # 3495036 by jonbok Thanks for the replies. I feel more confident now, and it looks like the engine is going to go back together very nicely.
Dick, that's a great procedure on setting the thrust washers. I would not have thought of that, and I did not see anything like that in the workshop manuals, but it makes perfect sense when you explain it. Nothing beats tapping into someone's experience.



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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